This question makes a classic causation/correlation mistake..twice!
It concludes that humans are able to support a large brain with a small gut likely due to the development of cooking. It then provides two correlations.
Firstly, that large brains began to develop around the same time as humans began to control fire (implying began to cook).
Secondly, people who only eat raw food have trouble getting sufficient calories. (Again, cooking correlating to more calories, and the lack of cooking correlating to less calories).
We need some information that will bolster this reasoning and provide evidence for cooking being the cause of this large brain, small gut characteristic.
Answer choice D says that it is more calorie expensive to consume raw food and by virtue of this, it would be more calorie efficient to consume cooked food. This supports the argument's conclusion.
Answer choice C states that there is no difference calorie-wise between consuming raw vs. cooked food. If this was the case, then how can the argument conclude that the efficiency of our calorie intake is due to cooking food? It can't. There must be some reason that makes eating cooked food more calorie rich, or eating raw food more calorie poor (answer choice D).
Hope this helped, please let me know if you have any other questions.