Which of the following describes a verb?

A verb is indeed defined as a word that indicates an action or state of being. This encompasses any action that a subject performs, such as "run," "jump," or "think," as well as states of existence, such as "is," "are," or "seems." Verbs serve as the backbone of sentences, providing vital information about what the subject is doing or experiencing.

Other options describe different parts of speech: the first option refers to adjectives, which describe nouns or pronouns; the second option pertains to pronouns, which replace nouns; and the last option again describes adjectives or adverbs that modify nouns or pronouns. Each of these functions plays a significant role in sentence structure, but only the definition provided aligns with the fundamental role of verbs in language.