This doesn’t capture the author’s belief that the results of the Ultimatum Game can be explained by the fact our emotions have been shaped by living in small groups. (A) is also not supported, because the Ultimatum Game involves only one example of a behavior that doesn’t seem to fit rational self-interest. This doesn’t establish that people do not make economic decisions out of rational self-interest. They could generally make such decisions out of self-interest, but with a few exceptions such as those decisions observed in the Ultimatum Game.

This is the best answer. It’s the only answer that captures the author’s belief that the results of the Ultimatum Game can be explained by the way we evolved. The “predisposition” is the tendency to respond angrily to unfair distributions of resources. This predisposition had “evolutionary value” in that it helped our ancestors survive.

This is part of the support offered for the author’s hypothesis that the results of the Ultimatum Game can be explained by the way we evolved. Although (C) is supported, it’s too narrow to be the main point.

This is the hypothesis in P3 that the author doubts. The author offers a different, “more compelling” explanation in P4. So the hypothesis from P3 can’t be the main point.

The author never suggests that the results of the Ultimatum Game are explained by a need to outcompete rivals. The idea of outcompeting one’s rivals relates to P3. But the hypothesis the author finds more compelling is in P4, and that hypothesis doesn’t involve the idea of outcompeting rivals.