(C) says, "The bodies of those who have the Apo-A-IV-2 gene excrete cholesterol when blood cholesterol reaches a certain level."
(C) is incorrect because all we know from the stimulus is that blood cholesterol levels of those with the Apo-A-IV-2 gene remained the same, whereas blood cholesterol levels of those with the Apo-A-IV-1 gene rose 20%. We don't have any information on how this happens. For all we know, maybe their bodies metabolized the cholesterol and converted it into fatty acids or some other type of biological substance. We don't have enough support to say that they excreted the cholesterol.
(E) says, "The presence of the Apo-A-IV-2 gene may inhibit the elevation of blood cholesterol."
(E) is strongly supported by the information in the stimulus, as the people in the study with the Apo-A-IV-2 gene were given a high-cholesterol food but had unchanged blood cholesterol levels. This suggests that the Apo-A-IV-2 gene may inhibit the elevation of blood cholesterol. (E) is the correct choice.
Does this make sense? Let us know if you have any other questions!