Interestingly, it looks like the author actually did get this one wrong.

I think her answers to several of the other questions are a bit obtuse - she's not actually unable to answer the questions, just showing how several answers could potentially relate while ignoring an obvious best. But her response here chooses C as 'obvious', then discusses how B could also apply. Even with a solid rationale for rejecting C, it's not a fantastic sign if a professional author addressing her own work genuinely gets the answer wrong.