The conclusion of the argument is: "accounting for this advance should not be counted as evidence in support of Einstein's theory."
Why? Because even though the advance of the perihelion of Mercury has been able to very simply and accurately predict Einstein's theory of general relativity, this theory was already well known when Einstein developed his theory and Einstein probably adjusted his equations to generate the correct numbers for the perihelion advance.
Answer choice (E) states that if a theory, i.e. Einstein's theory of general relativity, is adjusted to generate the correct predictions for some phenomenon, i.e. the perihelion of Mercury, that is already known to the scientists developing the theory, the theory should not be counted as predicting that phenomenon.
But, remember, the conclusion is that the advance of the perihelion of Mercury should not be counted as evidence in support of Einstein's theory. Answer choice (E) is saying that Einstein's theory should not be taken as evidence for the perihelion of Mercury, which is not the conclusion of the argument. Thus, answer choice (E) does not help justify the argument.
Answer choice (D), on the other hand, very clearly delineates what the argument is putting forth. If a theory, i.e. Einstein's theory, is adjusted specifically to account for some particular phenomenon, i.e. the perihelion of Mercury, a match between that theory and that phenomenon should not count as evidence in favor of the theory. Thus, answer choice (D) helps to justify the argument.
Hope that was helpful! Please let us know if you have any other questions.
