Which of the following is an example of an interjection?

An interjection is a part of speech that expresses emotion or exclamation and is often found in isolation from the main sentence structure. They typically convey strong emotions or sudden feelings and don’t require a grammatical connection to other parts of a sentence.

In this case, "Wow" clearly exemplifies an interjection. It conveys amazement or surprise and stands independently to express the speaker's feelings. When someone says "Wow," it instantly communicates a strong reaction without needing additional context or structure.

The other options do not serve this purpose. For instance, "Quickly" is an adverb that describes how an action is done; "Beautiful" is an adjective that describes a noun; and "Happiness," being a noun, denotes a state of being, rather than expressing an emotional outburst or exclamation. Thus, "Wow" effectively fits the criteria of an interjection, making it the correct choice.