Which of the following is an example of a noun?

The selection of "Dr. Carter" as the correct answer illustrates that it is a proper noun, specifically referring to a person by name. Proper nouns denote specific individuals, places, or organizations, distinguishing them from general nouns that refer to broader categories. In this case, "Dr. Carter" not only identifies a specific individual but also indicates a title associated with that person, indicating their profession.

The options presented illustrate different parts of speech: "quickly" is an adverb describing how an action is performed; "running" can function as a verb or a gerund (a noun formed from a verb), but it primarily denotes action rather than a discrete entity; "beautiful" is an adjective describing a noun but does not serve as a noun itself. The distinction of "Dr. Carter" being a specific and identifiable name categorizes it firmly as a noun in this context.