The correct answer, in my opinion, is C. worry. "Inform" has too neutral a meaning to be employed in this context. "Defame" means slander, denigrate, libel, smear - so it makes no sense in this sentence. On the other hand, "wary" is an adjective that means cautious, distrustful, suspicious, so it also makes no sense. Susan is just considerate, so she doesn't want to cause other people to worry because of her problems.