Which of the following correctly uses an adverb?

The choice that uses an adverb correctly is the one that describes how the mechanic worked by modifying the verb "worked." "Quickly" is the adverbial form of "quick," which means it properly indicates the manner of the action. In English, adverbs often end in "-ly" and serve to describe verbs, adjectives, or other adverbs. Therefore, saying "The mechanic worked quickly" accurately conveys that the mechanic performed his work in a fast manner.

In contrast, the other options either misuse adverbial forms or disrupt the sentence's natural flow. Using "quick" in the first choice is incorrect because it is an adjective, and adjectives do not modify verbs. The use of "quicklier" in the third option is grammatically incorrect since "quick" already has the comparative form "more quickly." The final option places the adverb "quickly" in an unusual position, which can create confusion, as it seems to imply that the adverb modifies "worked" rather than directly describing how the action was performed.