When we read the question, we see a long prompt, and a short stem, so we check out the stem. The phrase "most useful to determine" allows us to create a prediction immediately. The phrasing of Critical Reasoning questions tends to be understated. When you're asked what would be "useful to determine," look for what is critical to determine.
Applying the prediction, we look for a choice that must be true. (A) is out; we are told the system is effective, and people like it, and it's accessible. And they would pay for it! So it doesn't matter whether or not it's more effective. The prompt even tells us people prefer it to other forms of exercise. So (A) is out. (B) seems irrelevant; the system doesn't have to be for everyone in order to launch well. It's accessible to various types of people and they like it and so on. (B) is out. Choice (C) looks promising. Choice (D) contradicts the evidence. We're told it will take competitors a long time to copy this system, and we aren't concerned with the long term--we are talking about a "launch." Choice (E) is out, since the prompt doesn't say anything about cost.
We're left with (C).
We can look for logical confirmation of the correct answer: Is choice (C) critical? We can analyze by cases. What if it's hard to get people to try the new system? We could see that being an issue. But do we have objective backing in the prompt? Yes: we are told that potential customers liked the product "when they tried it during a two-month trial." What if they don't like the product yet after a month? Or more to the point, if it takes two months to like the product, and we are having trouble getting people to try it at all, that will impede the launch.
The correct answer is (C).