Complete Question Explanation
This stimulus represents another of the classic "some people say, but they're wrong, and here's why" structures, with the author telling us that some scientists believe the small humanoid skeletons they found on an island must be small because of a growth disorder. The conclusion to arguments that follow this pattern are almost invariably "but they're wrong", with the premises in support of that claim being the "here's why" portion, and the original claim of some people (the scientists here) being "a claim that the argument is designed to discredit" or something along those lines.
This stimulus follows that pattern to a T. The main conclusion is indeed that the scientists are probably incorrect, that rather than growth disorders the smallness of the skeletons is likely due to that species of humanoid shirnking over time from environmental pressures. The premises that support that conclusion are 1) the skeletons don't fit the typical pattern for growth disorders and 2) other animals on the island are also smaller than their off-island counterparts.
Our prephrase for this Main Point question, then, should be "those scientists are wrong, and the skeletons are probably those of a smaller species of humanoid."
Answer A: This gives us the claim of the scientists that the author seeks to discredit, the "some people say" part of the argument. That's not our Main Point and so is not the correct answer.
Answer B: This is the correct answer. This matches our prephrase and follows the "some people say, but they're wrong" format. The Main Point is that the skeletons are more likely to be of a small species of humanoid and not the result of growth disorders.
Answer C: This gives us one of the two premises that supports the main conclusion, not the main conclusion itself. While it is a true statement based on our stimulus, it is not the correct answer. Most Main Point questions have several of these "True But Wrong" answers that give us other parts of the argument than the one being asked about.
Answer D: Just like answer C, but this one gives us the second premise supporting our author's main point instead of the main point itself.
Answer E: This answer gives us an assumption of the argument. The author must believe that it is possible for environmental pressures to cause a species to shrink, otherwise they would not have built that very claim into their conclusion. The author never stated this directly, and she wasn't trying to prove this, but rather assumed this to be true as part of her support for the main conclusion, that this is in fact what likely happened in this case.