Hey there!
The answer is C. teenagers across the country.
A population is the whole. It's every member of the group, which is pretty much the opposite of a sample. A sample is just a fraction or a percentage of the group.
The sample is the homeroom class, who is the group being surveyed. But the population the survey is determining data for overall is teenagers across the country.
We also know that they are across the country because we are told that the gym is a national franchise.
Hope this helps!