Explanation

In the sentence "He walks fast," the word "fast" functions as an adverb, which is why option A is the correct choice. Adverbs are words that modify verbs, adjectives, or other adverbs, typically providing additional information about how an action is performed. In this case, "fast" describes the manner in which the verb "walks" is performedindicating that he walks quickly.

To further clarify, let’s analyze the structure of the sentence. "He" is the subject, "walks" is the verb, and "fast" modifies the verb "walks" by specifying the speed of the action. If "fast" were an adjective, it would modify a noun (for example, "a fast walker"), but in this sentence, it does not directly describe a noun; instead, it directly relates to the action of walking.

Thus, understanding the role of "fast" helps us identify it as an adverb. This distinction is crucial for grasping how different parts of speech function within a sentence, as it shapes our comprehension of the meaning and rhythm of the language.