Which of the following is an example of a noun?

The correct answer is "dog" because it is a noun that refers to a specific type of animal. Nouns are words that name people, places, things, or ideas. In this case, "dog" is a tangible entity that can be seen and interacted with, fitting the definition of a noun perfectly.

The other choices do not fulfill the criteria for nouns. "Running" is a gerund, functioning more like a verb when it describes the action of moving quickly on foot. "Beautiful" is an adjective that describes a quality or characteristic but does not name anything specifically. "Quickly" is an adverb that modifies verbs, adjectives, or other adverbs, indicating the manner in which an action is performed, rather than naming a person, place, or thing. Thus, they do not qualify as nouns, reinforcing why "dog" is the only correct choice in this context.