Whether people had previously read or listened to the novel doesn’t change the fact that most respondents who listened to it enjoyed it and most who read it didn’t. We can’t assume that their responses would’ve been different if they’d previously read or listened to the novel.

We can’t assume that the time it takes to read or listen to a book affects how much people enjoy the book. Thus, (A) doesn’t weaken the author’s argument, which relies on respondents’ reported enjoyment.

(C) doesn’t distinguish between readers and listeners; it only says that people are more likely to report enjoying the novel whether they read it or listened to it. But the author’s argument is about the difference in responses between readers and listeners.

This is true, but it doesn’t weaken the author’s argument. The fact that not all novels are available in audio versions does not challenge the conclusion that people people who listen to a novel are more likely to enjoy it than those who read it.

This weakens the author’s argument by highlighting the assumption that the survey results regarding this novel can be generalized to draw a conclusion about all novels. Just because this novel might be more enjoyable to listen to doesn’t mean all novels are.