Which part of speech primarily describes an action or a state of being?

The correct choice is related to how it represents actions or states. Verbs are fundamental in sentences because they convey what the subject is doing (an action) or indicate a condition or state of being (such as existence, possession, or change). For instance, in the sentences "She runs" and "He is happy," 'runs' denotes the action performed by the subject, while 'is' shows the state of being.

In contrast, nouns name people, places, things, or ideas, adjectives describe or modify nouns, and adverbs typically modify verbs, adjectives, or other adverbs, focusing more on how actions are performed rather than identifying the actions themselves or states of existence. Therefore, the role of a verb is central to articulating actions and states, making it the correct answer.