The federal rules aimed at protecting human subjects of medical experiments were established to ensure that patients
must be warned of potential risks and an independent panel would evaluate the experiment before it was conducted.
Option Elimination -
(A) must be warned of potential risks and an independent panel would - As RonTargetTestPrep pointed out, "ensure" and "must" convey the same meaning. Using both is redundant. The tense on both sides of AND needs to be parallel. "must be warned" - Present, "would evaluate" - Past. Wrong
(B) must be warned of potential risks and that an independent panel - "ensure" and "must" convey the same meaning. Using both is redundant. Both the events, the warning, and the evaluation, are future events said in the past, so "would be warned" - past and "would evaluate" - past is preferred.
(C) are warned about potential risks and that an independent panel - Both the events, the warning, and the evaluation, are future events said in the past, so "would be warned" - past and "would evaluate" - past is preferred. Also, "warned of" is preferred when indicating that someone is being informed or made aware of a specific danger, risk, or potential issue. For example, "Patients must be warned of potential risks" is a common and clear expression.
(D) will be warned about potential risks and an independent panel would - same as C.
(E) would be warned of potential risks and that an independent panel would - ok