There’s no suggestion the author would agree with (A), because the author isn’t trying to question the assumption that humans act primarily out of self-interest. Although the author discusses experiment results that are unexpected based on that assumption, the author does not draw the conclusion that the experiment shows that assumption is wrong. Since we don’t have any reason to think the author agrees with (A), it doesn’t fit at the end of the passage.

The word “unfortunately” doesn’t fit at the end of the passage, because the author never suggests that there is something undesirable about one-time, anonymous interactions.

This is the most logical completion, because it provides additional reason to think the hypothesis the author offers in P4 is a plausible explanation for the results of the Ultimatum Game. Remember, the Ultimatum Game involves people making offers and people accepting or rejecting offers. In P4, the author discusses how the way we evolved might explain why we reject low offers. But it never connects the way we evolved to why many people make fair offers. If the author believes the hypothesis in P4 is a more compelling explanation for the results of the Ultimatum Game than the hypothesis in P3, then that suggests the author believes the hypothesis in P4 also explains the way people make offers in the Ultimatum Game.

This doesn’t fit at the end, because we have no reason to believe the author thinks high self-esteem and a positive reputation offer “many other benefits” to people living in small groups. The author believes the way we evolved in small groups shapes our emotions. That’s the only way in which small groups are relevant to the author’s argument. Whatever other benefits might result from being in a small group don’t relate to the way our evolving in a small group explains the Ultimatum Game results.

This doesn’t fit at the end, because the author doesn’t show any concern for the question of what constitutes “fair.” The author cares about what explains why people behave the way they do in the Ultimatum Game, not with what kinds of resource splits are fair or unfair.