I really don't understand why C can't be an answer.

McKinley said the drug will have various effects on the patients' bodies. But he also said it will make us aware of "whether the patients are getting the drugs or a placebo" (Does it mean McKinley thought the placebo will produce no effects just like answer choice C?). So that's why I thought Engle said Mckinley assumed what the outcome of the study will be since McKinley said various effects of the drug AND whether the patients are getting the drugs or a placebo.

So can C be an answer?

Can anyone explain me why C can't be an answer and D is better?

Thanks!