A "Strengthen" question such as this must be approached in the same manner as one would a Logical Reasoning strengthen question, that is, you must identify the claim, the existing premises, and what kind of support might help the claim. Usually on these LR type reading comp questions, the reasoning itself will be more straightforward than on LR strengthen questions, but students do frequently have trouble with these. Some of the difficulty stems from the fact that students are unaccustomed to dealing with logical reasoning situations on reading comp. Therefore, your first order of business is to break the scenario down into the proper components.
The claim in Paragraph 3 is that the fair divisions common in the Ultimate Game stem from the long-term need to ensure a strong group, one in which all members contribute to each other according to their strengths. "From each according to his ability; to each according to his need" as it were.
The evidence that supports this claim is that it explains why the proposers would make equitable offers.
However, the author of the passage actually makes explicit which evidence is missing or unsatisfactory, i.e. this explanation makes no account of why others would reject low offers (ln 37).
To prephrase, you would need to find evidence that remedies this gap, you need to find a reason consistent with this "strong group" hypothesis that explains why responders reject the low offers.
Answer Choice B is not relevant. In fact, it alludes more to information either in paragraph 2 or paragraph 4. It is out of scope and does not help the claim in paragraph 3.
Answer Choice D provides "supporter" evidence that fills in the gap about responders' rejecting low offers. If it were equally counterproductive to the functioning of small groups for responders to reject low offers, then we have direct evidence that accounts for the information that the author found lacking, thus bolstering the "small group" hypothesis in paragraph 3.