ACT English Quiz: Sentence Structure And Formation
20 questions · exam conditions
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Sentence Structure And FormationQuestion 1 of 20

For the spring concert, the orchestra rehearsed in sections before playing as a full group. The string players worked on bowing together, while the wind players focused on matching tone. When rehearsal ended, the conductor thanked the students and reminding them to practice at home. On the way out, several students compared fingerings for a difficult passage. Which choice makes the sentence most grammatically acceptable?

When rehearsal ended, thanking the students, the conductor reminded them to practice at home.
NO CHANGE
When rehearsal ended; the conductor thanked the students and reminded them to practice at home.
When rehearsal ended, the conductor thanked the students and reminded them to practice at home.
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ACT English Quiz

ACT English Quiz: Sentence Structure And Formation

Practice Sentence Structure And Formation in ACT English with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Sentence Structure And Formation, giving you a quick way to practice the rules, question types, and explanations that matter most for ACT English.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

For the spring concert, the orchestra rehearsed in sections before playing as a full group. The string players worked on bowing together, while the wind players focused on matching tone. When rehearsal ended, the conductor thanked the students and reminding them to practice at home. On the way out, several students compared fingerings for a difficult passage. Which choice makes the sentence most grammatically acceptable?

  1. When rehearsal ended, thanking the students, the conductor reminded them to practice at home.
  2. NO CHANGE
  3. When rehearsal ended; the conductor thanked the students and reminded them to practice at home.
  4. When rehearsal ended, the conductor thanked the students and reminded them to practice at home. (correct answer)

Explanation: This question tests parallelism between verbs joined by "and." The conductor performs two past-tense actions, so both must take the same form: the version reading "the conductor thanked the students and reminded them to practice at home" pairs "thanked" with "reminded." The unchanged version follows "and" with "reminding," a participle that cannot serve as the second half of a compound predicate. The version placing a semicolon after "When rehearsal ended" repairs the verbs but punctuates wrongly, since a semicolon cannot follow an introductory dependent clause, where only a comma works. The version reading "thanking the students, the conductor reminded them" is grammatical, but stacking two modifiers before the subject buries the two equal actions that the parallel verbs state directly.

Question 2

Sarah enjoys hiking, biking, and to swim in her free time.

  1. NO CHANGE
  2. and to swimming
  3. also swimming
  4. and swimming (correct answer)

Explanation: The sentence lists three activities: 'hiking, biking, and   in her free time.' The rule is that every item in a series must take the same grammatical form, and because the first two items are -ing forms acting as nouns, the third must be one too, which makes 'and swimming' correct. Leaving the original 'and to swim' mixes an infinitive into a list of -ing forms, and 'and to swimming' is not a usable construction at all. 'Also swimming' has the right form but drops the coordinating conjunction that a three-item list needs before its final element. Read the first item in any list, then test each remaining item against it: if you have to change grammatical gears, that item is the error.

Question 3

At the community garden, volunteers planned a weekend workshop for new members. The schedule included a short tour, a demonstration of composting, and time for questions. To keep the event organized, the coordinator asked each volunteer to bring supplies and arrive early. The volunteers were asked to label seedlings, to set up chairs, and preparing signs. By noon, everything was ready for the first group. Which choice makes the sentence most grammatically acceptable?

  1. The volunteers were asked labeling seedlings, setting up chairs, and to prepare signs.
  2. NO CHANGE
  3. The volunteers were asked to label seedlings, and they set up chairs, and to prepare signs.
  4. The volunteers were asked to label seedlings, set up chairs, and prepare signs. (correct answer)

Explanation: This item tests parallelism in a list. When a single 'to' introduces a series of actions, every item must be a bare verb form, which is why 'to label seedlings, set up chairs, and prepare signs' is correct. Leaving the original wording mixes two infinitives with the -ing form 'preparing signs,' so the third item does not match the first two. The version beginning 'were asked labeling seedlings, setting up chairs, and to prepare signs' mixes -ing forms with an infinitive and also drops the 'to' that 'asked' requires, and the version containing 'and they set up chairs' breaks the list open with a full clause instead of a matching item. Read the shared lead-in word with each item in turn; if any pairing sounds wrong, that item is the one out of parallel.

Question 4

While I understand your concern, but I cannot change the decision.

  1. NO CHANGE
  2. however, I cannot change the decision
  3. I cannot change the decision (correct answer)
  4. so I cannot change the decision

Explanation: This item tests the rule against signaling one relationship twice. 'While' at the start of the sentence already marks the contrast between understanding the concern and being unable to change the decision, so no second contrast word belongs in the main clause; 'I cannot change the decision' completes the sentence cleanly. Keeping 'but I cannot change the decision' pairs a subordinating conjunction with a coordinating one, and 'however, I cannot change the decision' does the same job with a transitional adverb — both restate a contrast that is already established. 'So I cannot change the decision' is wrong for a different reason: it turns the contrast into a cause-and-effect claim, as if understanding the concern produced the refusal. When a sentence opens with although, while, though, or even though, expect the main clause to stand on its own with no contrast word attached.

Question 5

In science class, the teacher asked students to design a simple test of water quality in a nearby creek. Omar's group planned to measure pH, observe the types of insects they found, and record water temperature. Their plan was careful, thorough, and it was easy to repeat. The teacher approved it because another group could follow the same steps and compare results.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. Their plan was careful and thorough, and repeating it was easy.
  3. Their plan was careful, thorough, and easy to repeat. (correct answer)
  4. Their plan was careful, and thorough, and it was easy to repeat.

Explanation: Items in a series must share the same grammatical form, so a list that begins with adjectives has to end with an adjective. 'Their plan was careful, thorough, and easy to repeat.' keeps all three descriptions as adjective phrases attached to the single verb 'was,' which is why it is correct. Keeping the original ends the series with the full clause 'it was easy to repeat,' so the third item no longer matches the first two; the version reading 'Their plan was careful and thorough, and repeating it was easy.' abandons the three-item series for two clauses and adds words without adding meaning; and 'Their plan was careful, and thorough, and it was easy to repeat.' inserts a comma before 'and thorough' while leaving the mismatched clause untouched. When you spot a list, say the items aloud one after another and confirm they are all the same part of speech.

Question 6

At the library, a staff member recommended a novel to Kai, who had asked for something funny but thoughtful. Kai read the first chapter at a table near the window. The book was witty, and the characters felt real, and the plot moved quickly. He checked it out immediately and added the author to his list of favorites.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. The book was witty, and the characters felt real; the plot moved quickly.
  3. The book was witty; the characters felt real; and the plot moved quickly.
  4. The book was witty, the characters felt real, and the plot moved quickly. (correct answer)

Explanation: This question tests punctuation and parallelism in a series, here a series of three short clauses. The clean version is 'The book was witty, the characters felt real, and the plot moved quickly,' with commas between the items and a single 'and' before the last one. Keeping the original, 'The book was witty, and the characters felt real, and the plot moved quickly,' repeats 'and' where a comma alone would do, which makes the list plod. The version reading 'the characters felt real; the plot moved quickly' breaks the three matched items into an uneven two-part sentence, and the version with 'witty; the characters felt real; and the plot moved quickly' mixes semicolons with a conjunction even though these short clauses need no semicolons at all. For any list of three or more, use commas between items and reserve the conjunction for the final item.

Question 7

For her history project, Lila interviewed her grandfather about the neighborhood where he grew up. He described a movie theater that used to stand on the corner and a bakery that sold warm rolls every morning. She recorded the interview on her phone, she also took notes in a notebook. Later, she compared the details to old photographs from the library.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. She recorded the interview on her phone; she also took notes in a notebook. (correct answer)
  3. She recorded the interview on her phone also taking notes in a notebook.
  4. She recorded the interview on her phone, and she also took notes in a notebook, later.

Explanation: This question tests the correction of comma splices in compound sentences. The underlined portion is a comma splice because it joins two independent clauses with only a comma, without a coordinating conjunction or proper punctuation. Each clause has its own subject and verb, making them complete thoughts that cannot be connected solely by a comma. Choice B fixes this by replacing the comma with a semicolon, which is appropriate for joining two closely related independent clauses. Choice A retains the splice, C creates a run-on with an improper participial phrase, and D adds an unnecessary comma after 'notebook' and misplaced 'later.' A key strategy is to remember that two independent clauses cannot be joined with only a comma; use a period, semicolon, or comma plus coordinating conjunction.

Question 8

He enjoys playing basketball, watching movies, and to travel.

  1. NO CHANGE
  2. watch movies, and traveling
  3. to watch movies, and to travel
  4. watching movies, and traveling (correct answer)

Explanation: This question tests parallel structure in a list of activities. The original phrase mixes gerunds ('playing,' 'watching') with an infinitive ('to travel'), creating faulty parallelism. Choice D corrects this by making all three items gerunds: 'playing basketball, watching movies, and traveling.' Choice C makes all items infinitives but would need to include 'to play' as well for complete parallelism. Choice B mixes a bare infinitive with gerunds. For parallel structure in lists, all items must use the same grammatical form throughout - all gerunds, all infinitives, or all nouns.

Question 9

In history class, the teacher played recordings of speeches from different decades, and students compared how tone changes across time. Jordan took notes quickly, trying to capture repeated phrases and shifts in volume. He wanted to replay the last clip, he didn't want to miss the speaker's pause before the final line. After class, he asked for the link to the audio files.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. He wanted to replay the last clip: he didn't want to miss the speaker's pause before the final line,.
  3. He wanted to replay the last clip, and he didn't want to miss the speaker's pause before the final line,.
  4. He wanted to replay the last clip; he didn't want to miss the speaker's pause before the final line. (correct answer)

Explanation: This question tests the correction of a comma splice, where two independent clauses are incorrectly joined by only a comma. The original sentence has two independent clauses—'He wanted to replay the last clip' and 'he didn't want to miss the speaker's pause before the final line'—separated by a comma, which creates a run-on sentence. Using a semicolon properly joins the two closely related independent clauses without a conjunction. Choice D correctly uses a semicolon: 'He wanted to replay the last clip; he didn't want to miss the speaker's pause before the final line.' Choice A keeps the comma splice, Choice C adds an unnecessary comma at the end and uses 'and' improperly, and Choice B uses a colon incorrectly with an extra comma. A key strategy is to remember that two independent clauses can be joined with a semicolon, a period, or a comma plus a coordinating conjunction like 'and' or 'but.'

Question 10

At the community garden, volunteers rotate tasks each week. Some people water seedlings, and others pull weeds near the fence. Priya prefers harvesting because she can see progress quickly, and because she likes to share extra vegetables with neighbors. Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE (correct answer)
  2. because she likes to share extra vegetables with neighbors.
  3. she likes to share extra vegetables with neighbors.
  4. and she likes sharing extra vegetables with neighbors.

Explanation: This question tests parallel structure in compound phrases connected by coordinating conjunctions. The original sentence correctly uses parallel structure with two 'because' clauses: 'because she can see progress quickly' and 'because she likes to share extra vegetables with neighbors.' Both clauses are dependent and grammatically parallel. Choice B removes the coordinating conjunction 'and,' making the second clause a sentence fragment. Choice C removes 'because' from the second clause, breaking parallelism. Choice D changes the verb form to 'sharing,' which disrupts the parallel structure of the 'because' clauses. When items are joined by coordinating conjunctions, they must maintain parallel grammatical structure.

Question 11

Jorge trained for a 5K by running short routes during the week and a longer route on Saturday. He tracked his progress in a notebook, noting his time and how he felt afterward. Although he was tired after work, but he still ran. Over time, the routine became easier, and he began to look forward to the quiet stretch of road near his house.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. Although he was tired after work, he still ran. (correct answer)
  3. He was tired after work, although he still ran.
  4. Although tired after work; but he still ran.

Explanation: This question tests your understanding of redundant conjunctions. The original sentence incorrectly uses both 'Although' (subordinating conjunction) and 'but' (coordinating conjunction) to show contrast, creating redundancy. Choice B correctly uses only 'Although' to introduce the dependent clause, creating a proper complex sentence: 'Although he was tired after work' (dependent) + 'he still ran' (independent). Choice C places 'although' awkwardly in the middle, and Choice D creates multiple errors with a semicolon after a fragment and the redundant 'but.' Remember that you need only one conjunction to show contrast—either 'although/though' at the beginning or 'but' between clauses, never both.

Question 12

PASSAGE IV: Monopoly's Stolen Origins

¹ Almost everyone has played a game of Monopoly. With its iconic top hat playing piece, colorful paper money, and "Go to Jail" square, it is arguably the most famous board game in the world. According to popular corporate lore, the game was invented in 1933 by Charles Darrow, an unemployed heating engineer (31) who dreamed up the real estate trading game in his basement during the Great Depression. However, the true history of Monopoly is much more complicated, and it begins three decades earlier with a woman named Lizzie Magie.

² In 1903, Magie designed a board game she called The Landlord's Game. She was a progressive thinker and a follower of Henry George, an economist who believed that land should belong to everyone and that landlords unjustly profited from the labor of others. Magie created the game as an educational tool (32) to illustrate the negative consequences of land monopolies. (33) The board featured a continuous track around the outside, properties that could be bought, and a corner space that sent players to jail.

³ Magie's game was unique because it featured two sets of rules. Under the "Prosperity" rules, every player gained money when a property was acquired, reflecting the idea of shared wealth. Under the "Monopolist" rules, the goal was to acquire all the land and bankrupt the other players. Magie hoped that players would experience both versions and (34) have recognized the superiority of the cooperative approach.

⁴ Instead, players found the cutthroat "Monopolist" version to be far more thrilling. Over the next thirty years, the game spread organically through word of mouth. College students, Quakers, and intellectuals made their own handcrafted boards, modifying the rules and adding local street names. (35) One of these localized versions eventually found its way to Charles Darrow.

⁵ Darrow played the game, loved it, and asked his host for a written copy of the rules. He then made his own version, claiming the invention as his own, and sold it to Parker Brothers in 1935. It became an instant sensation, (36) making Darrow a millionaire. When Parker Brothers discovered that Darrow had not actually invented the game, they quietly bought the patent for The Landlord's Game from Lizzie Magie for a mere $500. (37) Magie never received any royalties from the millions of Monopoly games sold worldwide.

⁶ For decades, Magie's contribution was completely erased from history. It wasn't until a (38) high-profile legal battle in the 1970s that the true origins of the game were brought to light. Today, Lizzie Magie is finally being recognized as the brilliant, progressive inventor behind the world's favorite capitalist pastime. (39) Its a profound irony that a game designed to critique the concentration of wealth was ultimately used to make a few men very, very rich. (40)

Which of the following makes the sentence most grammatically acceptable?

  1. NO CHANGE — One of these localized versions eventually found its way to Charles Darrow. (correct answer)
  2. One of these localized versions eventually finding its way to Charles Darrow.
  3. One of these localized versions, eventually finding its way to Charles Darrow.
  4. Eventually finding its way to Charles Darrow, one of these localized versions.

Explanation: The correct answer is A (NO CHANGE). A is the only option that contains a complete, grammatically independent sentence. 'One of these localized versions eventually found its way to Charles Darrow' has a clear subject ('One of these localized versions') and a finite verb ('found') — it is a complete sentence. B ('eventually finding its way') replaces the finite verb 'found' with the present participle 'finding,' eliminating the main verb and creating a sentence fragment — there is no longer a complete predicate. C adds a comma before 'eventually finding' but the underlying problem remains: 'finding' is still a participle with no finite verb to anchor the sentence, making it a fragment. D ('Eventually finding its way to Charles Darrow, one of these localized versions') also uses 'finding' as a participle and has no finite verb — the sentence trails off without a predicate. All three wrong answers share the same fundamental error: replacing a finite verb with a participial phrase destroys the sentence's grammatical completeness. Pro tip: every sentence needs a subject and a finite (conjugated) verb. If you replace a conjugated verb with an -ing form and have no other finite verb, you have a fragment.

Question 13

The school newspaper ran a feature on students who commute in different ways. One student rode the bus, another walked, and a third biked along the river trail. The reporter asked about travel time, safety, and how much each student enjoyed to commute. She used the answers to compare the benefits of each route.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. The reporter asked about travel time, safety, and each student enjoyed commuting.
  3. The reporter asked about travel time, safety, and to enjoy each student's commute.
  4. The reporter asked about travel time, safety, and how much each student enjoyed commuting. (correct answer)

Explanation: This item tests parallelism: items joined in a series must share the same grammatical form. The reporter asked about three things, and since the first two are noun phrases ('travel time' and 'safety'), the third must also work as a noun-like object of 'about' — 'how much each student enjoyed commuting' does, because the whole clause names a topic and 'enjoyed' takes the gerund 'commuting.' The original 'how much each student enjoyed to commute' keeps the right clause shape but pairs 'enjoyed' with an infinitive, which that verb does not accept. 'And each student enjoyed commuting' drops the 'how much' frame and drops a full independent statement into a list of topics, and 'to enjoy each student's commute' shifts to an infinitive phrase that no longer matches 'travel time' and 'safety.' Read a series as 'asked about X, Y, and Z' and make sure each slot could stand alone in that frame.

Question 14

To prepare for the robotics competition, the team tested its design on the gym floor. The robot could turn sharply, but it sometimes drifted when it carried a heavy load. They adjusted the wheel alignment, the problem still appeared during longer runs. Finally, they rewrote the steering code to correct the drift gradually.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. They adjusted the wheel alignment; the problem still appeared during longer runs.
  3. They adjusted the wheel alignment, and the problem still appeared during longer runs. (correct answer)
  4. They adjusted the wheel alignment. The problem still appearing during longer runs.

Explanation: This question tests the correction of comma splices in compound sentences. The underlined portion is a comma splice, as it joins two independent clauses—'They adjusted the wheel alignment' and 'the problem still appeared during longer runs'—with only a comma, lacking a conjunction or proper punctuation. Both parts have subjects and verbs, making them complete thoughts that need appropriate connection. Choice C fixes this by adding 'and' after the comma, creating a proper compound sentence with a coordinating conjunction. Choice A retains the splice, B uses a semicolon which works but is not the marked choice, and D creates a fragment in the second sentence. To avoid comma splices, use a comma plus a coordinating conjunction like 'and' when joining two independent clauses that are related.

Question 15

In a geology unit, students examined rocks collected from a nearby riverbed. They compared textures, tested hardness, and looked for mineral streaks. While Elena labeled samples carefully, Marcus photographed them, and the teacher checked their charts. Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE (correct answer)
  2. Elena labeled samples carefully while Marcus photographed them, and the teacher checked their charts.
  3. While Elena labeled samples carefully, Marcus photographed them; the teacher checked their charts.
  4. While Elena labeled samples carefully. Marcus photographed them, and the teacher checked their charts.

Explanation: This question tests complex sentence structure with multiple clauses and proper coordination. The original sentence is grammatically correct, properly using a dependent clause ('While Elena labeled samples carefully') followed by two coordinated independent clauses ('Marcus photographed them, and the teacher checked their charts'). Choice B removes the subordinating conjunction, changing the meaning and structure. Choice C uses a semicolon incorrectly in place of proper coordination. Choice D creates a sentence fragment by placing a period after the dependent clause. Complex sentences with dependent clauses require proper coordination of any additional independent clauses that follow.

Question 16

In art class, students studied how light changes the appearance of objects. Ms. Chen set up a still life with a lamp, a glass bottle, and a folded cloth. She encouraged careful observation, sketching slowly, to shade with patience, and noticing reflections. Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. sketch slowly, shading with patience, and noticing reflections.
  3. sketching slowly, shading with patience, and to notice reflections.
  4. to sketch slowly, to shade with patience, and to notice reflections. (correct answer)

Explanation: This question tests parallel structure in a series of infinitive phrases. The original sentence breaks parallelism by mixing gerund phrases ('sketching slowly,' 'shading with patience') with an infinitive phrase ('noticing reflections') in a series meant to describe encouraged behaviors. Choice D maintains consistent parallel structure by using three infinitive phrases: 'to sketch slowly,' 'to shade with patience,' and 'to notice reflections.' Choice C mixes gerunds with an infinitive, maintaining the error. Choice B mixes different verb forms inconsistently. When listing actions or behaviors in a series, all items must follow the same grammatical pattern—all infinitives, all gerunds, or all imperatives.

Question 17

For a history project, Leo interviewed his grandmother about growing up near a railroad. He recorded the conversation, took notes, and later compared the details with newspaper archives. He wanted his report to be clear, to include vivid quotes, accurate dates, and explaining why the town changed. Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. including vivid quotes, accurate dates, and explaining why the town changed.
  3. to include vivid quotes, accurate dates, and to explain why the town changed. (correct answer)
  4. to include vivid quotes, accurate dates, and he explained why the town changed.

Explanation: This question tests parallel structure in infinitive phrases within a series. The original sentence breaks parallelism by mixing infinitive phrases ('to include vivid quotes,' 'accurate dates') with a gerund phrase ('explaining why the town changed'). Choice C maintains consistent parallel structure by using three infinitive phrases: 'to include,' 'accurate dates' (understood 'to include'), and 'to explain.' Choice B uses gerunds and participles inconsistently. Choice D shifts to a personal pronoun and changes verb form, breaking parallelism entirely. Items in a series must maintain the same grammatical structure—all infinitives, all gerunds, or all nouns.

Question 18

Ravi trained for a 5K by running three days a week. He increased his distance gradually and tracked his pace on a watch. He wanted to finish strong, and not only to avoid injury but also improving his time. Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. He wanted to finish strong; and not only to avoid injury, but also improving his time.
  3. He wanted to finish strong, and not only avoiding injury but also improving his time.
  4. He wanted to finish strong, not only to avoid injury but also to improve his time. (correct answer)

Explanation: This question tests parallel structure in correlative conjunctions and compound verb phrases. The original sentence incorrectly mixes infinitive and gerund forms in the 'not only...but also' construction: 'not only to avoid injury but also improving his time.' Choice D maintains parallel structure by using infinitive phrases in both parts: 'not only to avoid injury but also to improve his time.' Choice C adds an unnecessary coordinating conjunction 'and' before the correlative conjunction. Choice B uses incorrect punctuation and maintains the parallelism error. Correlative conjunctions like 'not only...but also' require identical grammatical structures in both parts of the construction.

Question 19

In a science lab, students tested how quickly sugar dissolves in water at different temperatures. They recorded the time for each trial and compared results. The warm water dissolved the sugar faster, but the cold water took longer. Afterward, the class discussed how temperature affects molecular motion.

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE (correct answer)
  2. The warm water dissolved the sugar faster, the cold water took longer.
  3. The warm water dissolved the sugar faster; but the cold water took longer.
  4. The warm water dissolved the sugar faster, and but the cold water took longer.

Explanation: This question tests the proper joining of two independent clauses with a comma and coordinating conjunction. The original sentence has two independent clauses—'The warm water dissolved the sugar faster' and 'the cold water took longer'—joined by a comma and 'but,' which correctly shows contrast and forms a compound sentence. No change is needed as the structure is already grammatically sound. Choice A is correct, maintaining the proper use of 'but' for contrast. Choice B creates a comma splice by removing 'but,' Choice C misuses a semicolon with 'but,' and Choice D redundantly adds 'and but.' Remember, two independent clauses cannot be joined with only a comma; use a comma plus a coordinating conjunction like 'but' for contrast.

Question 20

At the community center, volunteers sorted donated books into categories so families could browse easily. Some books had torn covers, but the pages were still intact. When the boxes arrived, and the coordinator checked the labels carefully. Then the volunteers placed the books on tables marked "Kids," "Teens," and "Adults."

Which choice makes the sentence most grammatically acceptable?

  1. NO CHANGE
  2. When the boxes arrived, the coordinator checked the labels carefully. (correct answer)
  3. The boxes arrived, and the coordinator checked the labels carefully.
  4. Arriving, the coordinator checked the labels carefully.

Explanation: This question tests the correction of a sentence fragment due to improper use of a subordinating conjunction and coordinator. The original sentence begins with the dependent clause 'When the boxes arrived,' followed by 'and' and an independent clause, but the 'and' disrupts the connection, resulting in an incomplete thought. Removing the 'and' properly attaches the dependent clause to the independent clause with a comma. Choice B correctly forms 'When the boxes arrived, the coordinator checked the labels carefully,' creating a complete complex sentence. Choice A keeps the fragment, Choice C changes it to two independent clauses but alters the temporal relationship, and Choice D creates a dangling modifier fragment. A useful strategy is to ensure that sentences starting with words like 'when' connect directly to an independent clause without unnecessary coordinators.