The conclusion of this argument is that when AG work becomes popular it's a sign that it's UNsuccessful.
Why? Because it doesn't fulfill the intentions of its creator (since beliefs don't change quickly), and we learn earlier that the goal is to change beliefs. (And, is it just me, or did the LSAT spell "fulfill" incorrectly there?).
(There are some issues with this argument, but finding those is not our job in this sort of question.)
The part we're asked about is the critic's claim that when an AG work becomes popular it is successful.
Seems like this statement runs directly counter to the argument, which is what (B) states. To be clear, while mentioning a counter point can be used in an argument to set the stage for what you're going to say (i.e., here's what I disagree with), you can't say that it's used to support the conclusion.
(A) is the opposite.
(C) is wrong because their claim isn't supported by anything. The initial premise is context, but not support.
(D) is mistaking the claim's role - it's not supporting the initial premise.
(E) is tempting since the claim is part of a counterargument, but it's a counterargument to the final claim, not the initial premise.
I hope that clears it up.
#officialexplanation