(A) doesn’t establish that if Selena doesn’t have psychic powers, no one else can have them. Sure, no one else has “yet been found” to have them. But that doesn’t mean it’s impossible for others to have psychic powers.

(B) establishes that if Selena doesn’t have psychic powers, then it’s impossible to have psychic powers (think about the contrapositive of this answer). This supplies the missing piece of the argument; if we find out Selena’s claim is not true, we can determine that it’s not possible to have psychic powers.

Whether it’s possible to determine that Selena’s claim is true/false has nothing to do with what happens IF we find out her claim is true/false. Adding (C) to the argument still leaves us not knowing whether other people besides Selena can have psychic powers.

We want to prove that by finding out whether Selena’s claim is true, we WILL determine whether it’s possible to have psychic powers. In other words, we’ll find out that it’s possible or that it’s impossible. (D) is directed toward proving that we “will not know whether it is possible” to have psychic powers. Not knowing whether it’s possible is the opposite of what we want to establish.

(E) simply reverses the conclusion. (E) establishes that finding out whether Selena has psychic powers is necessary to determine whether it’s possible to have psychic powers. But we want to establish that it’s sufficient to determine whether it’s possible to have psychic powers.