This must be true for the historian to infer that Robbins cannot pass judgment. If (A) weren’t true—if Robbins does not need to be capable of either dismissing or praising Stuart's art in order to pass judgment—then Robbins might be able to judge Stuart's art despite not being able to dismiss it or to praise it. In other words, if (A) weren't true, then the fact Robbins can't dismiss the art and can't praise the art wouldn't prove that she can't judge Stuart's art.

The conclusion is that Robbins can’t judge the art; there’s no claim about what people should do. The argument assumes that Robbins has failed to meet a requirement for being able to pass judgment on Stuart’s art.

The historian assumes that Robbins must understand the art well enough to praise it in order to pass judgment, not that she must be able to pass judgment on it in order to understand it. (C) gets the historian’s assumption backwards.

The historian establishes that Robbins can’t praise the art based on her current understanding, but it’s possible that other people can. The assumption is that Robbins’s inability to praise the art precludes her from judging it.

The argument is about Stuart’s art, not art in general. Also, the historian assumes that Robbins must be able to praise the art in order to judge it, not that she certainly will praise it if she understands it.