driving is fun.\nnoun\nverb\nadjective

Brief Explanations:

To determine the part of speech for "fun" in the sentence "Driving is fun", we analyze:

• A noun is a person, place, thing, or idea. "Fun" here isn't acting as a noun (e.g., "We had a fun" is incorrect; it would be "We had fun" where "fun" is more descriptive).
• A verb shows action or state of being. "Fun" doesn't show an action or a state of being in this context.
• An adjective describes a noun or pronoun. In "Driving is fun", "fun" describes the subject "driving" (telling us what driving is like), so it functions as an adjective.

Answer:

adjective