Confusing question, the age old negation method fails here.
Considering, 'So if we find out whether Selena's claim is true, we will thereby determine whether it is possible to have psychic powers.' as the conclusion under scrutiny.
A. No one else has yet been found to have psychic powers.
A can be correct. Since if someone already has psychic powers then finding out if Selena's claim is true does nothing about our ability to say whether psychic powers exist. If we negate A, we get that someone has psychic powers, so Selena's whole role in the argument/conclusion falls apart. We no longer need to determine Selena's claim to figure out if psychic powers are real. This assumption is essential to our argument.
B. If it is possible to have psychic powers, then Selena has them.
B can't be correct. Negating it we get something awkward. Selena has psychic powers and psychic powers are possible.
How I came to this?
https://www.math.niu.edu/~richard/Math101/implies.pdfSee page 3, example 5.
How is this hurting the argument?
C. It is possible to determine whether Selena has psychic powers.
Here in the stem, the conclusion is in the 'if..., then..' format. So this is not an assumption that affects the argument in anyway. Irrespective of our ability to determine Selena's ability, we can still make the argument that if we manage to gauge the ability then a certain thing will happen(in this case psychic powers will be proven)
Rest of the 2 are self explanatory.