• Why "relieving" is the correct answer:
Collocation: In English, certain words are "partners." The word stress is most commonly paired with the verb relieve. To "relieve stress" means to remove or reduce it, making a person feel better.
Positive Context: The sentence discusses the benefits of "leisure activities" (hobbies, sports, relaxation) on "physical and mental health." Since stress is a negative thing, a beneficial activity must do something positive to it—like reducing or removing it.
Parallelism: The sentence connects "relieving stress" with "offering a pleasurable experience." Both parts of the sentence describe positive outcomes.