It is a good question indeed.
Which of the following, if true, would best support the gym owner's argument?
A) Sports such as basketball and tennis also require shoes that are better designed for side-to-side motion. -> Irrelevant.
B) Cross-trainers provide as much cushioning as do running shoes. -> Cushioning is not the point gym owner is trying to make. Irrelevant.
C) Members of the owner's gym generally take advantage of all of the gym's programs. -> It looks odd first, but if members access different programs which requires cross training then, different type of shoes are required. Let's say, members don't have access to all programs and enrolled gym programs may require running, So, members don't require different shoes. Hence, accessing to all programs is vital for gym owner's argument to be true.
D) Most gym members who own cross-trainers also own a second pair of sneakers for everyday walking. -> Most of members have it. It never supports gym owner argument. Incorrect.
E) Proper footwear is the most significant factor in preventing injuries during exercise -> It is from premise, as doesn't extend support to gym owner's argument.
So, It is C.