Which of the following represents a verb form used as a noun?

The term that represents a verb form used as a noun is the gerund. Gerunds are formed by adding "ing" to a verb, which allows the verb to function as a noun in a sentence. For example, in the sentence "Swimming is fun," "swimming" acts as the subject of the sentence and is derived from the verb "swim." This transformation illustrates how gerunds bridge the gap between action and a substantive role in grammar.

In contrast, a noun refers directly to a person, place, thing, or idea and does not stem from a verb. Similarly, adjectives describe nouns, and adverbs modify verbs or adjectives, neither of which shifts into a noun form stemming from a verb. Thus, gerunds stand out as the correct choice for this question.