The phrase at the end of the sentence has two very different constructions for its objects, one simple description involving an object that is a noun ("it had a faulty battery"), and the other a verbal phrase ("the brakes were not working"). In a dependent clause with two objects, both need to have the same structure. The only answer choice with a correct parallel structure is "after which it had a faulty battery and non-working brakes."

The list that finishes this sentence must feature three similar items, so they can be properly grouped together. The list is written with two simple nouns ("art classes" and "language lessons") and one gerund phrase ("participating in sports"), meaning they need to be changed to matching items. The only answer choice that features a correct parallel structure is "between art classes, language lessons, and sports."

The list that finishes this sentence features unlike word forms in its items: it includes two gerunds ("breathing" and "balancing") and one infinitive ("to focus"). Any such list needs to feature words in the same form so that they can be properly grouped together, so the infinitive is not grammatically correct in this sentence. The only answer choice with an appropriate parallel structure is "as she deemphasized running and emphasized breathing, balancing, and focusing."

The list that finishes this sentence inappropriately groups together unlike items. It features two gerunds, "running" and "jumping," and the noun phrase "their ability to kick." Any such list needs to feature similar items that feature the same construction. The only answer choice that features a correct parallel structure is "and excelled at running, jumping, and kicking."

Any "either . . . or" phrase must feature similar items, so they can be properly grouped in the same phrase. However, the sentence is written so that its "either . . . or" phrase contains unlike elements, a noun ("specific benefits") and a gerund phrase ("working a set shift"). The only answer choice that features correct parallel structure in the phrase is "to have either specific benefits or a set work shift," which changes the gerund phrase to a noun so that the "either . . . or" phrase contains two nouns.

A list like the one that finishes this sentence needs to include three similar items, so that they can be properly grouped together. The sentence in this question, though, features two nouns ("design" and "expenses") and an infinitive phrase ("to fulfill its timetable"), meaning that the list needs to be changed so that all of its elements match. The only answer choice with a correct parallel structure is "it had problems in design, expenses, and timetable fulfillment," which changes the infinitive phrase "to fulfill its timetable" to a noun, "timetable fulfillment," that matches the other two nouns in the list.

In any "neither . . . nor" phrase, each item being compared has to be the same kind of item so a proper comparison can be made. The sentence is written with two different elements, the simple noun "antibiotics" and the verbal phrase "the use of disinfectants." The only answer choice that uses an appropriate parallel structure, by redeploying the verbal phrase to cover both items, is "Neither the use of antibiotics nor disinfectants helped slow."

The initial sentence is correct. The use of "either" in this manner requires that it be accompanied by "or."

"Chatting online" is a gerund phrase, while "video games" is a noun. In an "either . . . or" phrase, however, both elements of the phrase need to be of the same kind of word. "Either chatting online or playing video games might appear" is the only answer choice that uses an appropriate parallel structure.

Parallelism is the key here—all the descriptions should be in the same format, which is a single word adjective. The correct answer also makes the sentence more concise and more fluid, which is another benefit of having parallel sentence structure and another way to tell which answer choice is best.