Walking is a good exercise- Here `Walking` is-
Verb
Noun
Gerund
Correct
Adverb
Explanation:
Correct Answer is: Gerund
A gerund is a noun made from a verb root plus ing (a present participle). A whole gerund phrase functions in a sentence just like a noun, and can act as a subject, an object, or a predicate nominative.
Here walking came from the verb walk . Which is working as noun after ending with -ing.