Explanation

The word "substantiate" functions as a verb, which is the correct answer in the provided options. In linguistics, verbs are action words that express what the subject of a sentence is doing or what is happening. "Substantiate" specifically means to provide evidence to support or prove a claim, assertion, or theory. For example, in the context of a legal or academic discussion, one might say, "The researcher needs to substantiate her findings with data."

In contrast, the other options do not apply to "substantiate": an adverb (Option B) typically modifies a verb, adjective, or another adverb, indicating how, when, where, or to what extent an action is performed; an adjective (Option C) describes a noun and provides more information about it; and a noun (Option D) refers to a person, place, thing, or idea. Thus, since "substantiate" expresses an action and aligns with the grammatical definition of a verb, it confirms that the correct option is A) Verb.