So, in my opinion, even I chose D at first, but in reality the key role is played by the major word "sensitivity to.."
This is a causality based CR Question and therefore, it can be weakened if there is an alternate cause or reverse causality or not an alone cause.
We can rule out reverse causality but D actually is a shell game or trap option. It gives an alternate cause but when we choose it, we inherently assume that running is actually a cause of weight loss. (Trust me, it's so common that's why we assume it, IT IS NOT STATED IN QUESTION THAT RUNNING LEADS TO WEIGHT LOSS)
Now with B, it states sensitivity to wheat gluten. This shows that Ron was sensitive but not everyone who is reading his book. This destroys the conclusion because it breaks the assumption that all readers will be same as Ron. This assumption is clearly needed so as to prove that reduced wheat consumption will cause lower weight.
Therefore, B is better than D.
For A,C,E:
A is out because it's out of scope. We do not know if processed food is being talked about.
C is a close contender but again we assume that carbs are part of wheat or something. We can't ourselves assume unless stated so out.
E has nothing to do with the question. So out of scope.
Cheers!