Question

dancing is exercise.
noun
verb
adjective

Explanation:

In the sentence "Dancing is exercise", "dancing" functions as a noun (it's the subject, representing the activity as a thing). A verb would show action or state (but here "dancing" is not acting as a verb in the traditional verb - action sense in this sentence structure), and an adjective describes a noun, which "dancing" doesn't do here.

Answer:

noun