Which word is a noun?

The term "pen" is categorized as a noun because it refers to a tangible object that can be touched or used. Nouns are words that name people, places, things, or ideas. In this case, a pen is a common item used for writing, making it a concrete example of a noun in everyday language.

The other options do not fit the noun category. "Running" can be a gerund or a verb form, "quickly" functions as an adverb describing how an action is performed, and "above" is a preposition or adverb indicating a position compared to something else. These distinctions clarify why "pen" is the correct choice as a noun.