Hi, there! This is a tricky one, so I hope I can help you out.
Basically, the stimulus in this question is saying that the cause of Bach's lasting fame was the amount of music he produced, rather than the quality of his music overall. So writing lots of music causes fame. This is a cause and effect statement that you are looking to weaken it in the answer choices.
Answer (A) and (B) are very similar in this regard, so let's look a little closer.
Answer (B) states that there are highly regarded composers who wrote very little music. At first this is appealing, because it seems to contradict the stimulus that writing a lot of music leads to fame. However, the stimulus doesn't claim that writing a lot of music is the
only way to achieve fame--only that it is one cause of it.
Answer (A), meanwhile, states that there are musicians who produced even more music than Bach, and yet weren't famous. This is a direct violation to the cause/effect relationship in the stimulus: They wrote lots of music and still didn't get famous.
I hope that helps clarify things a bit!