Which of the following defines an adverb?

An adverb is defined as a word that describes a verb, and this is the reason why the selected answer is correct. Adverbs modify verbs by providing additional information about how, when, where, or to what extent an action is performed. For example, in the sentence "She runs quickly," the adverb "quickly" describes how she runs.

Additionally, it's important to note that adverbs can also modify adjectives and other adverbs. For instance, in "very fast," the adverb "very" modifies the adjective "fast." This multifunctionality highlights the role of adverbs in enhancing the meaning of verbs and other modifiers in a sentence.

In contrast, the other options illustrate different parts of speech: nouns are described by adjectives, conjunctions connect clauses, and prepositions show relationships between nouns, which are distinct from the function of adverbs. This distinction helps clarify why the correct choice explicitly defines the role of an adverb as a modifier of verbs.