Directions:- Choose the correct preposition from the given options to fill in the blank.

Lack of exercise can be harmful ______ one's health.

The correct phrase is "harmful for one's health." The preposition "for" is grammatically acceptable when discussing what is affected by something harmful. Both "for" and "to" are used with "harmful" depending on whether the focus is on what suffers (for) or what receives the harm (to). In this context, "for" correctly identifies health as the affected aspect.