Explanation

The word "gravity" is classified as a noun because it refers to a specific concept: the natural phenomenon by which all things with mass or energy are brought toward one another. In grammar, nouns are words that name people, places, things, or ideas. "Gravity" embodies this definition well since it names a fundamental force in physics that affects objects and bodies in the universe.

To clarify the other options:

A) A gerund is a verb form that ends in "-ing" and functions as a noun, like "running" in "Running is fun." "Gravity" does not fit this description.

B) A verb denotes an action or state of being; for example, "run" or "is." "Gravity" does not express an action.

C) An adjective describes a noun and typically modifies it, like in "heavy gravity." Again, "gravity" does not possess this descriptive role.

Therefore, "gravity" is most accurately identified as a noun, making option D the correct answer.