Which part of speech is used to express actions and states?

The correct choice is a verb because verbs are specifically designed to convey actions and states of being. They express what the subject of a sentence is doing (actions) or provide information about the subject's existence or condition (states). For example, in the sentences "She runs" or "They are happy," "runs" and "are" are the verbs that indicate an action and a state, respectively.

Understanding the function of verbs is essential in constructing sentences, as they serve as the core of the predicate, linking the subject to its action or state. This makes them fundamental to the structure and meaning of a sentence, allowing us to communicate dynamic processes and conditions effectively.