The conclusion is: "no one fundamentally desires anything except pleasure."
Why? We are told that pleasure is a natural result of getting what one wants.
So, the argument is reasoning that because pleasure is a natural result of getting what you want, there is no reason for someone to want something other than for pleasure.
But, just because pleasure is a natural result of getting what one wants, that doesn't mean that it's the only result or that people don't want other things. We are ignoring the middle ground here.
Our correct answer will show that A is a result of B, and will conclude that the only reason for B is A.
Answer choice (C) states: "Every time I eat pizza, I get a stomachache, so I suppose the reason I eat pizza in the first place is so that I can have a stomachache."
So, we are told that getting a stomachache is a natural result of eating pizza, therefore, the reason that this person eats pizza is to have a stomachache.
Clearly, we have the same reasoning as in the argument: A is a result of B, and so the only reason for B is A.
Hope that clears things up! Please let us know if you have any other questions.