Question

The marketing department of a sports supply store wants to estimate the percentage of adults in the town who belong to a gym.

Answer 1

A) If you only sample people in a sports supply store, your sample will be biased.  People in a sports supply store are more likely to be people that belong to a gym.
B) While it does not have necessarily the same amount of bias as sampling people in the sports supply store, people that go to a park are generally more likely to be people that exercise or have a gym membership.
C) Taking a random sample of people in town is a good way to get a non-biased sample.  They are not necessarily predisposed to answer your question one way or another.
D) For the same reason as choice A, this is a biased sample.

C is the best choice.