Which of the following describes how an adverb functions in a sentence?

An adverb functions in a sentence primarily to modify verbs, adjectives, or other adverbs, which often includes describing an action. For example, in the sentence "She runs quickly," the adverb "quickly" modifies the verb "runs," providing additional information about how the action is performed. This key role of adverbs in conveying specifics about actions makes option C the correct choice, as it highlights their main function in enhancing the meaning of verbs by explaining how, when, where, or to what extent an action occurs.

The other options do not accurately reflect the role of adverbs. A subject is typically a noun or pronoun that performs the action, which is not the function of adverbs. Modifying a noun is a function associated with adjectives, not adverbs. Replacing a noun is also not a role that adverbs play; this function is typically related to pronouns. Thus, the distinct characteristic of adverbs as modifiers of actions confirms that to describe an action is indeed their primary function.