Which of the following words is NOT a preposition?

The word "quickly" is identified as the correct choice because it is an adverb, not a preposition. Adverbs modify verbs, adjectives, or other adverbs, providing additional information about how, when, where, or to what extent something occurs.

In contrast, the other words—"before," "about," and "with"—are all prepositions. Prepositions are words that establish relationships between nouns (or pronouns) and other words in a sentence, often indicating direction, location, time, or possession. For example, "before" indicates a time relationship, "about" provides a topic of discussion, and "with" indicates a relationship between entities. Therefore, understanding the roles of these word classes helps to correctly identify which word does not fit as a preposition.