Choice C is the best answer. The main purpose of the fifth paragraph (lines 57-65,"But...athletes") is to relate what Maguire discovered about the mental athletes, namely that their brain structures are not different from those of the control group but that the mental athletes use their brains differently: "there was one telling difference . . . regions of the brain that were less active in the control subjects seemed to be working in overdrive for the mental athletes."
Choice A is incorrect because the fifth paragraph does not mention the taxi drivers or the study involving them. Choice B is incorrect because the fifth paragraph describes some of the unexpected results of Maguire's study but does not address the possible reasons for those results. Choice D is incorrect because the fifth paragraph describes only Maguire's findings, not her methods.