"Lassitude" and "lethargy" both mean apathy or fatigue. "Diligence" means perseverance in carrying out an action. "Vigor" means power or energy. "Largess" means generosity. "Legerdemain" means sleight of hand. 

"Exigency" and "contingency" both mean difficulty or crisis. "Boon" means an advantage or blessing. "Hegemony" means dominion or authority. "Transient" means temporary or ephemeral. "Largess" means generosity or aid.

"Atavism" and "reversion" both mean backsliding or regression. "Pariah" means social outcast. "Pathos" means deep sadness or emotion. "Polemic" means argument or spat. "Rectitude" means uprightness or decency.

"Lien" and "claim" both mean a claim on property. "Consensus" means general agreement or accord. "Suffragist" means a voter or citizen. "Neologism" means a new word. "Whelp" means a young animal, especially a mammal.

"Commandment" is an official order usually by a government official. "Dearth" means lacking of something. "Compassion" means showing emotion and sympathy for another person. "Insomnia" means sleeplessness or inability to sleep. "Rebellion" means a revolt against a government or some other entity.

"Enigma" means puzzle or something difficult to understand. "Accreditation" means providing credit or approval for an institution (usually a school). "Zealot" is a person who is passionate for something. "Fatigue" means exhausted and worn-out. "Herbivore" is an organism that eats only plants.

"Felicity" means bliss or extreme happiness. "Inconsistent" means not constant or not the same throughout. "Frightful" means horrible or causing fear in people. "Humid" means that there is a lot of moisture in the air. "Inactive" means not moving or not involved with physical activity.

The word “alacrity” is related to the musical words “allegro” and “allegretto,” which indicate quickness or briskness of tempo. Strictly speaking, “alacrity” means pleasant, quick readiness (to do something). Among the provided options, the best answer is “willingness.”

"Zenith" means apex or highest point, so "apex" is the correct answer. None of the other answer choices are close in meaning to highest point: "zealot" means fanatic or enthusiast; "reconciliation" means agreement or harmony reached after an argument or battle; "cessation" means ending or conclusion; and "vulgarity" means crassness or obscenity.

"Mettle" is a noun that means "courage; fortitude" or "the ability to cope well with difficulties; resilience." While "diffidence" might seem like a promising answer choice, it actually means "modesty or shyness resulting from a lack of self-confidence," so it's an antonym of "mettle," not a synonym. "Dauntlessness," on the other hand, means "fearlessness, intrepidness, boldness; the incapability of being discouraged or intimidated" and is the best answer choice of those listed.