To justify the author's reasoning we need to find a FLAW in the approach mentioned.
(A) Any object that is posited by a scientific theory and that enhances the explanatory power of that theory should be designated as real.
--Correct! This is in opposition of the approach mentioned in the paragraph and, if true, would imply that the approach was flawed.
(B) Objects posited for theoretical reasons only should never be designated as real.
--Wrong. The paragraph states that,"most scientific theories contain entities posited solely on theoretical grounds". This option doesn't pose a flaw in the approach but questions the validity of "Most of the Scientific theories'.
(C) A scientific theory should not posit any entity that does no enhance the explanatory power of the theory.
--Wrong. It actually supports the approach.
(D) A scientific theory should sometimes posit entities on grounds other than theoretical ones.
---Out of scope.
(E) Only objects posited by explanatorily powerful theories should be designated as real.
--Wrong. Paraphrases the approach.