Hi rjhyman!
You've got it! The reason why (A) is incorrect is because it does not weaken the conclusion the author is making. Whether or not Lichtenstein attended exhibitions by abstract expressionist painters in the 1960s makes no part of the author's characterization of Lichtenstein more or less likely!
Additionally, the assertion that Lichtenstein attended exhibitions by abstract expressionists does not necessarily weaken the statement that "abstract expressionism had lost much of its force." Perhaps there were only 5 abstract expressionism exhibitions per year (compared to something like 100 in prior years). Well, even if Lichtenstein attended all/some of those exhibitions, it does not change the fact that abstract expressionism lost force (which, in this example, is exemplified by the vast reduction in exhibitions).
Now, whether or not (A) weakens the statement about abstract expressionism losing its force is ultimately irrelevant to the question being asked here, since we're looking to weaken "the author's characterization of
Lichtenstein", not abstract expressionism! So, (A) is out of the scope and we can eliminate this answer choice.
I hope this helps
Kate