Hi Lsat180Please,
Good catch on the wording! The conflation of relative qualities with absolute ones ("aesthetically better" vs "aesthetically pleasing") is a commonly tested subject on the LSAT, and test takers should always have it in the back of their minds whenever a comparison between two things is made in a stimulus. That is what is being tested here, not as a flaw, but as a necessary assumption to make the argument work. This leads to a powerful prephrase: in order for us to know the abstract impressionist paintings are "pleasing," rather than just better (but still bad!), we have to assume the preschoolers' paintings are good enough that anything better is automatically above the "aesthetically pleasing" threshold.
Only (B) does this, and we can test it out by using the Assumption Negation method:
Most of the preschoolers' painting were aesthetically displeasing
abstract expressionist paintings are not necessarily aesthetically pleasing
This works perfectly, confirming that (B) is the correct answer choice.
Hope this clears things up!