Question

swimming is an exercise that strengthens endurance.
participle
gerund
infinitive

Explanation:

A gerund is a verb form ending in -ing that functions as a noun. In the sentence "Swimming is an exercise that strengthens endurance", "Swimming" acts as the subject (a noun - like role), so it is a gerund. A participle (present participle also ends in -ing) functions as an adjective or part of a verb phrase, and an infinitive is in the form "to + verb", so neither of those fit here.

Answer:

gerund