The correct answer to this question is Gerund. In this sentence, swimming is the verb, and this verb is a gerund. It is a gerund because it is used as a noun. It is also a gerund because it ends in -ing. Gerunds are not to be confused with present participles, which even end in -ing.
The difference between the two is that gerunds are a noun throughout the more significant part of the sentence. Examples include "Playing this game makes the day go by faster." Another example is "Watching this baby brings joy to my day." This type of question would be found on an English test.