Please explain

This effectively asks us what must be true in order for the argument to make sense. So. We are looking for something that must be true in the context of the argument, something the author must believe. The author doesn't have to believe c. Perhaps a book on moral philosophy should in fact contain beliefs one disagrees with, in order to rebut them or expose one's audience to other views. Either way, it isn't really relevant to the plagarism issue here, the problem isn't whether they are beliefs the author holds, the issue is whether the author credited the other person. However, the author must believe that d is true. The author thinks it was wrong not to credit the other person. However, if the other person actually got their ideas from the author in questions, then the author owes the other person no intellectual debt.