(A) leads to the wrong conclusion. We aren’t trying to prove whether Einstein’s theory discovered Mercury’s orbit. We just want to know if predicting it is evidence that the theory is correct.

Wrong trigger. (B) translates to—if a theory is not developed with a phenomenon in mind, then predicting that phenomenon is not good evidence that the theory is correct. We know that the theory was developed with Mercury’s orbit in mind, so we can’t use this conditional statement.

Wrong trigger and wrong conclusion. We don’t know whether Einstein’s theory was able to accurately account for phenomena other than Mercury’s orbit, so we can’t use this conditional statement. We also aren’t trying to prove that Einstein’s theory as a whole isn’t well supported, just that its prediction of Mercury’s orbit does not constitute support.

We know from the premises that Einstein adjusted his theory to match the orbit of Mercury. (D) tells us this means the theory’s prediction of Mercury’s orbit is not evidence that the theory is correct, which justifies our conclusion.

(E) leads to the wrong conclusion. We want to prove that predicting Mercury’s orbit is not evidence that the theory is correct, not that the theory didn’t predict Mercury’s orbit.