What part of speech expresses excitement or emotion?

The correct choice is interjection, as interjections are specifically designed to express emotions or sudden exclamations. They often stand alone and can convey strong feelings such as surprise, joy, sorrow, or emphasis. Common examples include words like "Wow!", "Oh no!", and "Hooray!". These words do not function grammatically within a sentence like other parts of speech; rather, they serve as a burst of emotion, often punctuated by an exclamation point.

In contrast, adjectives describe or modify nouns, adverbs modify verbs, adjectives, or other adverbs, and prepositions indicate relationships between nouns (or pronouns) and other words in a sentence. While adjectives, adverbs, and prepositions play crucial roles in sentence structure, they do not inherently express emotional responses in the same way that interjections do. Thus, interjections stand out as the part of speech dedicated to conveying emotion and excitement.