This is a sufficient assumption question, meaning the correct answer choice when combined with the rest of the premises guarantees the truth of the conclusion.
The argument tells us that if we find out whether Selena's claim is true, we will determine whether it is possible to have psychic powers, which ignores the possibility that Selena's claim could be false, and it is still generally possible to have psychic powers. The correct answer choice needs to provide this missing link - that if it is at all possible to have psychic powers, then Selena has them and then we could logically conclude that if Selena has them, we can determine whether it is possible.
The issue with (D) is that even if it is true, the conclusion does not logically follow. It only tells us what happens if the claim turns out to be false, there is no connection to the conclusion which stipulates what happens when the claim is true.
The issue with (E) is that it reverses the conclusion
It can be diagrammed as:
able to determine - > find out the claim is true, whereas the conclusion is: