I think that answer choice B is still incorrect and answer choice C is correct.
The music critic's conclusion is about the characteristics of success, claiming that sales are not an indicator. So then, an underground group not meeting criteria that other groups find desirable should not be considered a marking of their success (ie their recording may not sell well and the group is still considered successful). But then the critic states that group's low sales could just mean that the group is incompetent (ie that they are not actually very good and that is why they have low sales).
For his/her conclusion to be valid, we just need to justify that a group may have low sales and still be successful (answer choice C). For all we know, incompetence is also part of the the definition of success in underground groups. Why should we assume that if they are incompetent and have low sales that they are not successful? I know it sounds funny but I think in LSAT world, it's a stretch to assume test taker equates competence and success (especially since, in this question, we are not even equating selling more recordings with success).
Does anyone else agree?