Concept:A gerund is a verb form that ends in "-ing" but functions as a noun, such as the subject of a sentence.
Explanation:The sentence given is "Swimming is good for health."
In this sentence, "Swimming" is the subject of the verb "is."
A subject is normally a noun or a pronoun.
"Swimming" is formed from the verb "swim" by adding "-ing," yet it is placed in the subject position.
This makes it a gerund.
It is not a present participle, because it does not describe a noun or form a continuous tense like "He is swimming."
It is not the main verb, because the main verb here is "is."
It is not an ordinary noun, because it comes directly from a verb and keeps its "-ing" form.
Therefore, "Swimming" is being used as a gerund in this sentence.
Answer:D. Gerund