20. (E)
Question type: Assumption
According to the argument, if medicine can effectively treat high blood pressure, then some cases of high blood pressure must not be caused by stress, since those cases could only be impacted by a reduction of stress. But this conclusion assumes that the medicine itself has no affect on stress-reduction. Answer (E) clearly states this assumption.
(A) is not an assumption required by the argument.
(B) is not an assumption required by the argument.
(C) is tempting because it relates stress reduction and medication, but consider negating this assumption. If it is said that reduced stress does impact the effectiveness of the medication, would that undermine the conclusion? In fact it would have no bearing and the conclusion that some cases of high blood pressure have causes other than stress would still stand.
(D) is incorrect because it essentially restates the conclusion.