Ok, I will be as concise as possible.
I easily identified the last sentence as the conclusion and the first two as support. From that I got the argument to be (in a nutshell) that stretching did not make a difference because the group that stretched and the group that didn’t had the same number of injuries anyway. Thus, the medical researcher fails to consider that stretching may have indeed had an impact on the first group; perhaps they would have been injured much more had they not stretched.
I should mention that looking at the question again, and taking my time to understand it without a time constraint, really allowed me to clearly define and lay out the flaw on paper without just holding it in my head for the answer elimination process. On the real exam, this would be too long of a process to go through, I think.
In any case, answer D still doesn’t do it for me. I can try to wrap my head around it, but it does not explicitly state that stretching will have a positive outcome on the joggers. I find myself having to make that additional inference for myself and thus taking more time to sift through each answer to make sure I did not miss anything.