After staring at this question for too long and reading too many "explanations" about why B clearly isn't right (it's just barely not right), here's what's actually happening. A few other comments address this too, but just in case you missed them:

We're trying to support the idea that the researchers failed to find ANY correlation between pain intensity and the weather. The correlation between beliefs about the weather and pain intensity, as we see in B, is still some type of loose correlation. C simply says that the correlation doesn't actually exist at all, hence why they conclude that the experiment failed to find any correlation.

I think what makes this question hard is that they draw an awful conclusion from the results of their experiment. The researchers are refuting the idea that placebos can have real effects, which we in the real world know just isn't true. But we still have to fill in the blank as they would, and they clearly don't think that any correlation is evidenced by their experiment.

If more people chose A than chose D... doesn't that empirically suggest that A is more strongly implied?

I see now why D is right, but Engle's response would be exactly the same if he thought McKinley was presuming that the placebo will produce no effects (AC C).

M: The tested drug will produce various effects (but the placebo certainly will not).

E: McKinley, you fool, you're assuming you already know the outcomes of your study. Maybe the placebo WILL produce effects. Therefore, you CAN'T conclude that a double-blind study is impossible.

So I'm really really not seeing how we can eliminate C.

Off topic comment, this stim is an excellent example of why you should always avoid analogy in technical writing.

A target time of 7 minutes on this passage is absurd.

Wait, so flints highly polished for religious use DO indicate an aesthetic sense, but flints highly polished for household use DON'T? That requires assuming that some religious practice is motivated by aesthetics, which.... huh?

"More fully" = "more consistently"? Ehhhhh..... that's a stretch.