The idea that certain objects “enhance the explanatory power of a theory” doesn’t connect to any of the concepts in the premise. We know from the premise that most scientific theories contain entities posited solely on theoretical grounds. But we don’t know whether these entities enhance explanatory power. So (A) doesn’t strengthen the argument.

(B) builds a bridge from the premise to the conclusion. We know from a premise that most scientific theories posit some entities solely on theoretical grounds. (B), then, allows us to infer that these entities shouldn’t be considered real. So if an approach would designate these entities as real, that approach must be flawed.

(C) is directed toward telling a scientific theory that it shouldn’t posit a certain kind of entity. But we’re trying to show that certain entities posited by theories shouldn’t be designated as real; we’re not trying to show that certain entities shouldn’t be posited.

(D) helps support a conclusion that a scientific theory should sometimes posit certain entities. But we’re trying to show that certain entities posited by theories shouldn’t be designated as real; we’re not trying to show that scientifies theories should posit certain entities.

(E) would allow us to infer that objects that are not posited by explanatorily powerful theories shouldn’t be designed as real. But the premise doesn’t establish that we’re dealing with any scientific theories that are not explanatorily powerful. So (E) doesn’t interact with the premise and therefore doesn’t strengthen the argument.