The theory is (let's call it R): " millions of sweat glands in human skin evolved as a way to keep humans cool while running over long distances or for extended periods. ". Then it says that R is based on (let's call it Q): "this ability developed as a hunting strategy that depended on (let's call it P) out-enduring prey by pushing animals to overheat".
So we have a P that is causing Q, which is in turn causing R. (P -> Q -> R)
By asking "One of the first things that must be shown," the question is asking us to prove whether P is correct. IF it is not, the Q will fail, and hence R will fail. Option B (OA) indicates that P is correct.
But I think there is problem here.
R is dependent on Q, but not necesarily on P. If P doesn't exist, but some other factor X exists which is causing Q, then theory R will still hold true, and will be acceptable. R is only dependent on Q, not anything else. X -> Q -> R.
Think about it!! how can whether animals can be pushed to a point where overheating will make them easier prey, or not, will make any impact on the evolution of sweat glands in humans to keep them cool?? There is no logical connection.
Let's say animals can NOT be pushed to a point where overheating will make them easier prey. Still sweat glands may have evolved due to running over long distances, even if this particular hunting strategy had failed. Evolution of sweat glands is dependent on running over long distances, and hunting could be a valid reason. But it has nothing to do with overheating of animals, making them easy prey.
I understand that the question is trying to test us on our ability to understand the cause-effect P -> Q -> R, but I think the example that has been chosen is poor quality.