Which part of speech would describe the manner in which an action is performed?

The correct choice describes the manner in which an action is performed because adverbs are specifically designed to provide details about verbs, adjectives, and even other adverbs. They often answer questions such as "how?", "when?", "where?", and "to what extent?". For example, in the phrase "She sings beautifully," the adverb "beautifully" describes how she sings, providing important context about the action.

In contrast, nouns name people, places, things, or ideas; verbs represent actions or states of being; and adjectives describe nouns, giving additional detail about them. Thus, the role of adverbs in expressing the manner of an action is what makes this answer the most fitting choice.