Happy to help. Let's take a look at (A), (C), and (D). We're looking for the answer that tells us the primary purpose of the passage.
(A) says, "survey existing interpretations of the puzzling results of an experiment"
The author of the passage examines just one interpretation of the perplexing results of the Ultimatum Game. (A)'s wording, in saying "surveying," suggests far more diligent investigations of the existing interpretations. Additionally, the passage's author is far more concerned with presenting her own interpretation. Thus, (A) does not capture the primary purpose of the passage, so it's out.
(C) says, "argue that the results of an experiment, while puzzling, are valid"
The passage's author never says that the Ultimatum Game's results are valid. The results are accepted, and their potential invalidity is never even brought up. The author simply wants to explain the results because they appear to have a conflict with a number of economic assumptions (lines 22-27). Thus, (C) is factually inaccurate, so we can get rid of it.
(D) says, "offer a plausible explanation for the puzzling results of an experiment"
In reviewing the passage, we see that the author is seeking to give an answer to a question. She is providing a plausible explanation for the perplexing results (the rejection of low offers) in an experiment, and this is her reasoning for writing the passage. Thus, (D) captures the main point really well, so it's the correct answer choice.
Does this make sense? Let us know if you have any more questions!
