- Which of the following is Caledonia's strongest constitutional argument?
Answer: The Act commandeers state executive officials by compelling them to administer a federal regulatory inspection scheme, which the Tenth Amendment does not permit.
When you see a federal statute directing state officials to enforce a federal regulatory scheme, your mind should go to the Tenth Amendment's anti-commandeering doctrine. Here, Caledonia's strongest argument is that the Act commandeers state executive officials: it compels state public utility commissions to conduct inspections, prepare federal-form reports, and refer violations to a federal agency. Under cases like New York v. United States and Printz v. United States, Congress may not conscript state executive officials to administer federal regulatory programs. That is precisely what this Act does, so the anti-commandeering challenge is the most powerful.
The invalid-delegation argument misses the issue: Congress can delegate federal inspection authority, but whether state commissions are responsible to the President or have consented is not the constitutional flaw — the flaw is compulsion itself, not lack of federal control or consent. Similarly, the Commerce Clause argument is weak: natural gas pipelines are economic and interstate in nature, and Congress may regulate intrastate activities that substantially affect interstate commerce; the Act's safety protocol easily fits within that power. Finally, the sovereign-immunity argument is misplaced: state sovereign immunity protects states from private suits, not from federal enforcement actions brought by the United States, and an "unfunded mandate" is not itself the core constitutional violation — the Tenth Amendment commandeering principle is.
On exam questions like this, spot "state officials forced to implement federal law" and immediately test whether the statute commandeers them; that is your winning argument.
- Is the State Q franchise tax valid as applied to the Federal Reserve Bank and to the national bank?
Answer: It is valid as to the national bank because the tax applies equally to state-chartered banks, but invalid as to the Federal Reserve Bank because federal law bars state taxation of Federal Reserve banks.
This question tests intergovernmental immunity and, more importantly, Congress's power to consent to state taxation of federal instrumentalities. Whenever you see a tax on a federally chartered entity, first ask: did Congress authorize this tax, and did the state follow Congress's conditions? The Federal Reserve Act is the exclusive source of authority here, and it draws a sharp line.
Federal law flatly bars state taxes on Federal Reserve banks except taxes on their real property. Therefore the State Q franchise tax on net capital is invalid as applied to the Federal Reserve Bank, regardless of whether the tax is nondiscriminatory or whether the bank engages in commercial activities. In contrast, Congress expressly allows states to tax national banks, but only if the tax is imposed in the same manner and at the same rate as taxes on state-chartered banks and is not discriminatory. State Q's franchise tax applies at the same 1% rate to state-chartered banks, so it satisfies those conditions. Thus the tax is valid as to the national bank but invalid as to the Federal Reserve Bank.
Now the traps. The choice saying the tax is invalid as to both because Congress has not expressly consented gets the national bank half wrong: Congress expressly consented. The choice saying it is valid as to both because states may tax all financial institutions nondiscriminatorily overstates state power; Congress prohibited the tax on Federal Reserve banks. The choice saying the Federal Reserve tax is valid because it conducts commercial activities confuses factual activity with statutory immunity, and the national bank is not absolutely immune, as the same choice claims. Your takeaway: for federal instrumentalities, the first question is not general tax policy, but what Congress has permitted.
- Is the Bond Interest Revenue Act's tax on interest from Madison's bonds valid?
Answer: No, because the Act taxes interest on state bonds at a higher rate than on comparable private bonds, thereby discriminating against state bonds in violation of intergovernmental immunity.
Whenever you see a state bond combined with a federal tax, think intergovernmental immunity. The key distinction is between a nondiscriminatory tax, which is generally allowed even if it raises state borrowing costs, and a discriminatory tax, which is invalid. In Carver, the Court made exactly that point: a federal tax on state bond interest may burden the state, but it cannot single out state bonds compared with substantially similar private bonds.
Here, the Act imposes a 25% tax on interest from Madison's bonds but only a 20% tax on "all other bonds," which would include comparable private bonds. That rate differential is precisely the discrimination Carver forbids. So the tax is invalid, regardless of whether Madison can prove a specific financial impact.
The choice saying Congress may impose any tax as long as it falls on bondholders rather than the state is too broad: holder-level taxes still cannot discriminate against state bonds. The choice saying the tax is valid because it has a legitimate federal revenue purpose misses the point—a legitimate purpose does not cure discriminatory treatment. And the choice claiming any federal tax on state bond interest is an unconstitutional interference goes too far; nondiscriminatory bondholder taxes are constitutional.
Study tip: on intergovernmental immunity questions, check whether the tax applies equally to comparable private activity. Equal treatment usually passes; a special burden on the state fails.
- Which state(s) may constitutionally tax Lee's federal salary?
Answer: Only State A, because Lee is a resident of State A and, under the Act, a teleworking federal employee performs services at his actual work location, which is in State A rather than State B.
This question tests the intersection of intergovernmental immunity and congressional consent: states may generally not tax federal employees' salaries unless Congress authorizes the tax, so your first step is always to read the consent statute. Here, the Federal Employee Tax Act is the exclusive consent, and it allows a state to tax a federal employee only if the employee is a resident of that state or performs services there. Critically, it defines a teleworker's regular place of work—and thus where services are performed—as the actual work location, not the federal agency's office. Lee lives in State A and works from his home in State A, so State A may tax him as a resident and as the state where he actually performs services. State B may not: although its own law treats teleworkers as working in State B, that state-law characterization is preempted by the federal statutory definition, and Lee is not a State B resident. The answer suggesting both states may tax under general nondiscriminatory jurisdiction misses that Congress made this Act the exclusive consent and that the Act limits State B's reach. The answer choosing only State B because the employing agency is located there reverses the Act's explicit telework rule. And the answer claiming neither state may tax absent a separate waiver ignores that the Act itself is the waiver and that it clearly permits State A to tax its resident. On bar questions, remember: federal employees are not wholly immune—locate Congress's consent, then apply its precise residency and work-location rules.
- Which of the following is the most likely result?
Answer: The tax is valid because the legal incidence of the tax falls on Resource's privately owned equipment, and the tax is nondiscriminatory and generally applicable.
Whenever a question involves a contractor working with the federal government, focus on the legal incidence of the tax, not its economic burden. The constitutional immunity of the federal government is narrow: it protects the United States itself, not every private business that happens to deal with it. Here, Resource owns the equipment, and the State taxes all business equipment generally. The legal incidence falls on Resource as a private taxpayer; the fact that the Department of Energy reimburses Resource's costs is only an economic consequence, not a legal one. Because the tax is nondiscriminatory and generally applicable, it is valid.
The argument that the tax is invalid because the practical burden falls on the federal treasury confuses economic impact with legal incidence. A state may not tax the federal government directly, but it may tax a private contractor, even if the contractor passes the cost along. The argument that the equipment is federal property simply because it sits on federally owned land ignores ownership: Resource owns the equipment, and location alone does not transform private property into federal property. Finally, there is no basis for saying the federal government has expressly consented to all such taxes—consent is unnecessary because the tax falls on a private party, not on the federal government.
Study tip: when you see a claim that a tax "burdens" the federal government, ask who has the legal obligation to pay. That determines the outcome.
- Which of the following is the strongest basis for upholding the federal statute?
Answer: The condition is a valid exercise of the spending power because it is unambiguous, reasonably related to highway safety, and leaves the State with the choice to forgo 10 percent of its highway funds.
When you see a federal spending-condition challenge, think of South Dakota v. Dole: Congress may attach conditions to funds if they are unambiguous, related to the federal program, and do not coerce states. Here, withholding 10% of highway funds unless the drinking age is 21 is a valid spending-power exercise—the condition is clear, it plainly relates to highway safety, and Altura retains the choice to reject the funds rather than raise its drinking age. That choice is what defeats the state's "commandeering" argument: a condition is not a command.
. The agency-discretion argument fails because a statute's constitutional validity cannot be insulated by committing enforcement to agency discretion; nor does that answer whether Congress acted within its power. The "any objective" theory fails because the spending power is not unlimited—it must be exercised for the general welfare and conditions must be related to the federal interest, so an unrelated condition would not stand. The Twenty-First Amendment point is also backward: that Amendment reserves broad alcohol-regulation authority to the States and did not grant Congress concurrent power to set a minimum drinking age; the statutory condition instead relies on the spending power. So the strongest basis is the familiar Dole analysis: unambiguity, relatedness, and voluntariness. On exam day, when a state challenges a funding condition, ask: Is the condition clear? Is it related to the program? Does it leave the state a real choice? If yes, the condition likely survives.
- Is State L's tax on the Postal Service's leasehold estate valid?
Answer: No, because the tax's legal incidence falls directly on the United States as the holder of a property interest, and the federal government's property interests, including leaseholds, are immune from state taxation despite the commercial character of the lease.
Whenever you see a state tax challenged on intergovernmental immunity grounds, the key is identifying the legal incidence of the tax—who is legally required to pay—not who benefits or whether the tax is evenhanded. Here,the tax is assessed to and payable by "the holder of the leasehold estate." Because the USPS—the United States—holds that leasehold interest, the legal incidence falls directly on the federal government.
Morrison resolves the question: the United States' property interests, including leasehold interests, are constitutionally immune from state taxation; a state tax whose legal incidence falls on the United States is invalid regardless of commercial terms or whether the lease is called commercial. So the tax is invalid even though private leaseholds are taxed the same way.
The choice saying the tax is valid because it targets the leasehold estate anyway osition tax is valid because it targets the leasehold estate, not the fee, and the lease was voluntarily acquired--misses that a leasehold itself is a property interest of the United States, and immunity attaches to that interest, not just fee ownership. The choice arguing states may tax all leasehold interests identically, including federal ones, mistakes nondiscrimination for immunity: the federal government is not impliedly subject to state taxes merely because private parties are taxed similarly. Finally, the choice claiming thefederal government waived immunity by entering into a commercial lease on market terms is directly contrary to Morrison, which holds that entering into commercial leases does not waive immunity.
Your takeaway: always ask "On whom does the legal incidence rest?" If it rests on thefederal government or its instrumentality, state tax is invalid—even if the lease is commercial, even if private parties are taxed identically.
- Which of the following is the strongest basis for reversing the dismissal?
Answer: The state statute is invalid because a State may not close its courts to federal claims that Congress validly assigns to them;the anti-commandeering doctrine does not protect state judiciaries.
Whenever you see a state court refusing to hear a federal claim, think about the Supremacy Clause and the ordinary rule of concurrent jurisdiction: unless Congress gives federal courts exclusive jurisdiction, state courts must hear federal causes of action. Here Congress did more than create a right — it expressly directed state courts of general jurisdiction to entertain Telephone Privacy Act claims. The Supreme Court has long held that state courts must honor such a congressional assignment; they are not being "commandeered" because federal law binds state judges through the Supremacy Clause. Winslow's statute closing its courts to all newly created federal claims absent implementing legislation directly conflicts with that duty and is invalid. So reversing the dismissal is required.
The Tenth Amendment argument gets anti-commandeering backwards: New York and Printz restrict Congress from commandeering state legislatures and executive officials, not state judiciaries. The claim that Congress has exclusive jurisdiction over all federal claims is also false; federal question jurisdiction is normally concurrent with state courts. And the Necessary and Proper Clause argument does not support reversal — Congress has not made this jurisdiction exclusive; it has assigned these cases to state courts. N&P is not what invalidates a state's refusal to hear them; the Supremacy Clause is. Watch for the pattern: "state courts must hear validly assigned federal claims" paired with "anti-commandeering does not protect judiciaries" is the reliable trigger.
- Which of the following is the most significant constitutional issue raised by the states' challenge?
Answer: Whether the federal income tax on interest earned from state bonds violates the constitutional doctrine of intergovernmental immunity.
This question tests intergovernmental immunity, the implied constitutional limit on how the federal government can tax state and local governments. When you see a federal tax that applies specifically to state government activity, your first instinct should be to ask whether the tax impermissibly burdens state sovereignty. Here, Congress taxed interest on state and local bonds while leaving federal bond interest exempt, and the states argue that this raises their borrowing costs. That argument is fundamentally an intergovernmental immunity challenge: does the federal tax interfere with the states' ability to function as sovereign governments? Historically, the Supreme Court has held that the federal government may tax state bond interest unless the tax discriminates against states, so this is the central legal question.
The Tenth Amendment argument is not as strong because the tax does not commandeer the states. Commandeering occurs when the federal government forces states to enact or administer federal programs, not when it merely changes the tax consequences of state borrowing decisions. The Dormant Commerce Clause is also misplaced because that doctrine limits state laws discriminating against interstate commerce, not federal tax choices between state and federal bonds. Finally, the Guarantee Clause claim fails because that clause is generally nonjusticiable and does not guarantee states any particular level of financing for public services.
Study tip: When a federal law affects state governments, separate three common arguments—intergovernmental immunity, Tenth Amendment commandeering, and federalism-based limits—and ask whether the law directly commands states or merely imposes an indirect financial burden.
- Is Delta's toll revenue subject to the federal excise tax?
Answer: Yes, because the tax is nondiscriminatory, and Delta's operation of toll roads is a commercial activity also engaged in by private entities, not a core sovereign function.
Whenever a state argues it is immune from a federal tax, remember the modern intergovernmental-immunity rule: immunity is narrow. The federal government may impose a nondiscriminatory tax on a state when it engages in a commercial activity that private actors also conduct for profit, but it may not impose a tax directly on core sovereign functions like courts, tax collection, or elections.
Here, Delta's toll roads are commercial enterprises, and private companies also operate toll roads in Delta. The federal excise tax applies alike to state and private operators, so it is nondiscriminatory. Under Franklin, operating toll roads is not a core sovereign function merely because the state does it under statutory authority or for a public purpose. Therefore, Delta's toll revenue is taxable.
The choice saying "No, because toll roads are public improvements operated for a public purpose" confuses public purpose with sovereign immunity; commercial enterprises are taxable despite serving the public. The choice saying "No, because the federal government may not impose any tax directly on a state or its instrumentalities" repeats the old absolute-immunity doctrine, which the Supreme Court has rejected. Finally, "Yes, because Congress may tax states whenever the tax is enacted for the general welfare and applies evenhandedly" reaches the right result but is too broad: even an evenhanded tax could be unconstitutional if it directly targets a core sovereign function.
On exam day, check two things: Is the tax nondiscriminatory? Is the state activity a commercial enterprise rather than a core sovereign function? If both point to "yes," the state pays.
- Which of the following is the most likely basis for holding the Act unconstitutional?
Answer: The Act commandeers state and local executive officials to administer a federal regulatory program in violation of the Tenth Amendment.
Whenever you see a federal law telling state or local governments to do something, your mind should jump to the anti-commandeering doctrine under the Tenth Amendment. This is a core principle distinguishing permissible federal regulation from impermissible conscription of state machinery. Here, the Act requires state and local law-enforcement agencies to conduct background checks and report to the FBI, backed by a civil fine. This is a textbook case of commandeering state executive officials to administer a federal regulatory program, which the Supreme Court struck down in Printz and New York v. United States. That is why the correct basis for holding it unconstitutional is that it commandeers state and local executive officials.
Now look at the distractors. The choice claiming the Act exceeds Congress's commerce power because it regulates state governments rather than private commercial activity is a trap: Congress can regulate state governments under the Commerce Clause, but it cannot commandeer them. The problem is the method, not the subject matter. The choice about an unfunded mandate is also wrong—the Tenth Amendment does not prohibit unfunded mandates; the issue is the command to implement, not the lack of funding. Finally, the spending power choice is incorrect because this is not a voluntary grant program; it is a coercive fine. The Spending Power allows conditional grants, but a direct fine is an exercise of regulatory power, not a voluntary offer.
Study tip: Distinguish "commandeering" (telling states to regulate) from "preemption" (federal law taking over) and "spending conditions" (offering money with strings). If a federal law forces state officials to act, it's likely unconstitutional.
- Who prevails on TDC's refund claim?
Answer: State X prevails, because the legal incidence of the tax falls on TDC and the tax applies generally to all defense-related businesses without singling out federal contractors.
Whenever you see a state tax challenge involving a federal contractor, the key question is: where does the legal incidence of the tax fall? Intergovernmental immunity protects the federal government itself, not every private party that happens to do business with it. Here, the tax is imposed on "any person engaged in the design or manufacture of defense-related equipment," so the legal obligation rests squarely on TDC. The cost-reimbursement contract shifts the economic burden to the Navy, but as the Court explained in Acme Industries, passing costs along does not transform a private contractor into an immune federal instrumentality. Because the tax applies generally to all defense-related businesses—not just federal contractors—it does not discriminate against those doing business with the United States. State X therefore prevails.
TDC's first argument fails because it focuses on the economic burden rather than the legal incidence; that is the classic trap this precedent rejects. The claim that TDC is a "federal instrumentality" by performing a vital federal function is also wrong—merely contracting with the government does not confer sovereign immunity. Finally, the argument that the tax "singles out" defense work is a misread: it targets a broad category of state businesses, and TDC is taxed as a state actor, not because it works for the Navy. The fact that the federal government ultimately pays does not make the tax one on the United States.
Study tip: On bar questions, always separate who is legally liable from who pays the bill. Legal incidence controls intergovernmental immunity; contractual cost-shifting does not.
- Which of the following is the strongest basis for holding the University liable?
Answer: Because the Act is a generally applicable labor law regulating the State's own conduct as an employer, and the Tenth Amendment does not exempt States from such neutral federal laws.
Whenever a state challenges a federal law under the Tenth Amendment, ask whether the law commandeers the state's regulatory machinery or simply regulates the state's own conduct as an employer. Under Garcia v. San Antonio Metropolitan Transit Authority, the Tenth Amendment does not shield states from generally applicable, neutral federal labor laws that apply to the states themselves. The University is the state acting as an employer, so Congress could validly require it to pay the minimum wage. Thus, the strongest basis is that the Act is a generally applicable labor law regulating the state's own conduct as an employer.
As for the wrong answers: the first choice, claiming the University "cannot claim Tenth Amendment protections that a State itself could assert," gets the doctrine backward — a State itself would also lack protection here. The "Commerce Clause gives Congress exclusive authority over all employment relationships" is overbroad; Congress's power is broad, but not exclusive, and many employment relationships remain subject to state regulation. Finally, the "spending-power condition" explanation fails because the Act is a direct regulation, not a condition attached to federal research funding; the University's receipt of funds is irrelevant.
A study takeaway: when you see a Tenth Amendment challenge, ask whether the federal law imposes obligations on the state as a sovereign regulator or merely regulates the state's own conduct as a participant in the economy. The former is often invalid as commandeering; the latter is generally valid.
- Which of the following is the strongest ground for invalidating the tax?
Answer: The tax discriminates against federal retirees by exempting comparable state retirement benefits while taxing federal retirement benefits.
Whenever you see a state tax that treats federal employees differently from state employees, your first instinct should be intergovernmental tax immunity. Under the modern doctrine, states may tax federal employees, but they cannot discriminate against them or the federal government. Here, the statute exempts state retirement benefits but taxes federal pension benefits. That is textbook discrimination against federal retirees. The Supreme Court in Davis v. Michigan Dept. of Treasury held such a tax violates the implied constitutional immunity, because it places a heavier burden on federal retirees than on state retirees. The legal incidence of the tax is on the retiree, not the government, but the discriminatory structure is the fatal flaw.
The first choice, that the tax is on the United States because pension funds are appropriated by Congress, is wrong because the legal incidence falls on the retiree, not the federal treasury. The Tenth Amendment argument fails because the issue is not a lack of congressional consent; it is the discriminatory nature of the tax itself. Finally, the Privileges and Immunities Clause is irrelevant—that clause protects out-of-state citizens from discrimination, not discrimination between two classes of in-state residents.
For exam day, remember the "discrimination trap": a state can tax federal pensions, but only if it taxes state pensions equally. If you see an exemption for state benefits, it is automatically suspect under Davis.
- Which statement best describes whether the taxes are valid?
Answer: The gross receipts tax is valid, while the property tax is invalid, because the legal incidence of the gross receipts tax falls on OroTech, whereas the legal incidence of the property tax falls on the United States as the laboratory's owner of record.
Whenever you see an intergovernmental-immunity question, focus on one thing: which party bears the legal incidence of each tax under state law? Labels, federal supervision, and economic burden do not control. The Supreme Court's Draven Systems statement is the whole test.
The gross receipts tax is valid. Its statute imposes a 2% tax on "every person doing business in this State"—OroTech is a private corporation doing business there, so the legal obligation falls on OroTech. It is an ordinary, nondiscriminatory tax on a private contractor, and it is irrelevant that OroTech operates a federal lab, is called an"instrumentality," or passes the cost to the United States through its cost-plus contract. The property tax, however, is invalid. The statute taxes"all real property... assessed to and payable by the owner of record."The United States owns the laboratory and is the owner of record, so the legal incidence falls on the United States, not OroTech. OroTech's contractual promise to pay the tax on the United States' behalf does not shift legal incidence; it is exactly the kind of agreement the Court said does not alter state-law incidence. Thus a state may not compel the United States to pay a property tax on federal property.
The"both taxes are invalid" position fails because it treats supervision, the"instrumentality" label, and the United States' economic burden as enough—but the doctrine protects only the legal incidence, not he who ultimately pays. The"both taxes are valid" position fails as to the property tax because OroTech's agreement to pay does not make it the taxpayer; the statute itself names the owner of record, the United States. And the"gross receipts invalid, property valid"position has it backwards: a state may not tax federal land itself, but it may tax a private contractor's gross receipts despite federal supervision.
So remember: economic burden and contractual labels are red herrings. Ask simply, "Whom does state law require to pay?" That answer decides immunity.
- Which of the following is the most significant legal issue raised by the institute's protest?
Answer: Whether the institute's federal contract makes it a federal instrumentality entitled to intergovernmental immunity from the state gross receipts tax.
Whenever you see a state trying to tax money flowing from the federal government to a private contractor, think of intergovernmental immunity. The question is about the institute's protest over the federal contract payments, so the core issue is whether that contract makes the institute a federal instrumentality entitled to immunity from the state gross receipts tax. That is the most significant legal issue, and it is governed by the standard from United States v. New Mexico: a contractor is not entitled to immunity merely because it operates a federally funded laboratory under a renewable cost-reimbursement contract; it must be so closely connected to the government that it is in effect acting as the government itself. The distractor about the economic burden falling on the U.S. Treasury is the classic trap—the Supreme Court rejected the economic burden test, holding that the possibility of passing the tax on to the federal government does not create constitutional immunity. The choice involving the dormant Commerce Clause and out-of-state patent licensing is less significant because the institute's protest is specifically directed at the federal payments, not interstate commerce. The preemption choice is also off base: no federal statute preempts the state tax, and the proper constitutional framework is the implied immunity doctrine, not statutory conflict preemption. So the instrumentality issue is the key. On exam day, when you see a state tax on a federal contractor, ask whether the contractor is truly a federal instrumentality—and do not fall for the economic-burden trap.
- Under these principles, is the Labor Commissioner's prevailing-wage order enforceable against BuildRight?
Answer: Yes, because the state law is a generally applicable, nondiscriminatory measure that does not conflict with the federal wage regulations, which set only minimum rates.
Whenever you see a state law applied to a federal contractor, remember that intergovernmental immunity is narrow: a generally applicable, nondiscriminatory state law survives unless it actually conflicts with federal law. The Hamilton quote states this rule directly. Here, State Y's prevailing-wage statute applies to all contractors on public improvements, so it is generally applicable and does not target the federal government. The incorporated federal wage rules set only minimum rates, so a higher state prevailing wage does not conflict with them. Even though complying costs BuildRight more, the Court said that alone does not invalidate the law. Therefore, the Labor Commissioner's order is enforceable.
The wrong answers each distort the doctrine. "Any state regulation that increases the cost of performing a federal contract" is too broad — the question specifically rejects cost alone as fatal. "Federal procurement is exclusively governed by federal law" assumes preemption, but there is no actual conflict here. And calling BuildRight a "federal instrumentality immune from state regulation" is wrong: federal contractors generally are not immune merely because they work for the government.
On exam day, when you see federal contractor plus state regulation, ask two questions: Does the law discriminate against the federal government? Does it conflict with a specific federal law? If the answer to both is no, the state law applies.
- Which of the following is the most likely reason the tax is invalid?
Answer: The tax discriminates against the federal government and those with whom it deals, which the doctrine of intergovernmental tax immunity prohibits.
Whenever you see a state tax challenged by someone connected with the federal government, your first thought should be intergovernmental tax immunity. That doctrine says a state may not use its taxing power to burden, discriminate against, or control the federal government or those with whom it deals.
here, State L's tax falls on federal employees' gross salaries and on no other residents' salaries. That singling out is the constitutional problem: it is not a neutral income tax that happens to include federal workers; it is a targeted financial burden on the federal employ relationship. Such discrimination against the federal government and its employees is exactly what intergovernmental tax immunity prohibits, so the tax is invalid.
Now the wrong answers:
The Article IV Privileges and Immunities Clause protects nonresidents against discrimination in basic rights; it does not protect a resident federal employee against his own state's occupancy tax. So that choice misses the doctrine and the class protected.
Equal protection would apply rational-basis review here, and a state likely could justify classifying federal employees differently for some legitimate purpose; constitutional law, however, does not permit discriminating against federal employment on this basis. Thus equal protection is not the most likely grounds for invalidity.
Finally, the Supremacy Clause argument fails because Congress need not expressly consent to every state tax on federal employees; instead, the doctrine of intergovernmental immunity is the constitutional limit, and the question is whether the tax discriminates against the federal government, not whether Congress said "yes."
Study tip: On bar-exam tax-immunity questions, look for a tax that appears neutral but is actually aimed at federal employees or federal contractors. The key word is usually discrimination — a targeted tax on the federal relationship violates intergovernmental immunity, while a generally applicable tax may be upheld.
