Not necessary, because as long as people can accurately judge the aesthetics of a painting when comparing it with another painting, it doesn’t matter whether their ability to judge the aesthetics is better or worse in some other situation. The study happened to involve presenting pairs of paintings; maybe presenting only 1 painting at a time could be a better way of judging aesthetics? That doesn’t affect the reasoning, because it can still be possible to judge aesthetics of paintings when they are presented together.

Necessary, becaues if this were not true — if half or more of the preschoolers’ paintings used in the study WERE aesthetically displeasing — then the fact that most participants in the study judged the abstract expressionist painting as “better” would not justify the conclusion that abstract expressionist paintings are in the category of “aesthetically pleasing.” The negation of (B) leaves open the possibility that the abstract expressionist paintings could have been better, but still displeasing.

If anything, (C) would hurt the argument, because if the paintings were labeled, that could present an alternate interpretation of the study. Maybe people were just rating the abstract expressionist painting as better because they were biased against preschoolers’ paintings?

Not necessary, because the argument is based on the people who DID consistently rate the abstract expressionist paintings as better. Whatever is true about the group that did NOT rate abstract expressionist paintings as better doesn’t affect our interpretation of the results of the people who DID consistently rate the paintings as better.

Not necessary, because even if (E) were not true — if there were a large number of stylistic similarities — we still know that most participants rated the abstract expressionist painting as better. A painting can still be aesthetically better than another, even if there are many similarities between the two.