The point at issue between Patricia and Tamara is whether or not most Japanese feared ninjas.
(B) is wrong because it does not address this issue. If ninjas learned how to walk on floors quietly, this would mean that the squeaky floors were ineffective but it would not mean anything about the existence of fear or lack thereof.
(C) is correct because it points out that the wealthy are only a small part of the population. Therefore, Tamara's objection would not destroy Patricia's argument because most Japanese could still be unafraid of ninjas. The rich may simply make up the minority that are afraid.
Does that make sense? Let us know if you have any other questions and best of luck with your studies!