Its D for me.
The study is about "efficacy" or effectiveness of the drug.
The 1st speaker is merely saying that the effects of the drug on the group will be different from the effects of the placebo on the other group. He is not necessarily saying that the drug will have its INTENDED effects (therapeutic) on the group. Infact, that is what the study is done for! To find out the effect of a drug on a patient in comparison to "no treatment (placebo group). If the 1st speaker were to imply that, then he would not be saying that its not possible to do the study. Infact, the study would have been successfully concluded!!
What the 1st speaker is implying is that other unintended effects of the drug such as, side-effects of the drug would distinguish the treatment group from the placebo group even before the conclusion of the study. Therefore, it would no longer remain a double-blind study.
The 2nd speaker is misinterpreting the 1st speaker's statement as just that. He is thinking that the 1st speaker has already assumed that the drug is going to treat the group's symptoms and therefore, this group will obviously look different from the placebo group. Well that's what we want at the end of the study to say that the drug is efficacious or effective in treating the illness, but that is not what the 1st speaker is saying.