In Homesick: My Own Story by Jean Fritz the reason why Jean tell Lin Nai-Nai not to use "sewing machine" as an English greeting at the end of the story is C: because he doesn't want Lin Nai-Nai to embarrass herself.
This fictionalized autobiography relates the story of a young girl ( the author) immigrating to China in the middle of political agitation. Lin Nai-Nai is her nanny and becomes an important support for her. This little girl notices that Ian Forbes, a villain according to the author's view, has tricked Lin Nai-Nai into saying something silly and tells her nanny right away since she is really fond of her and does not want to see her ridiculizing herself in public.