Tollens' test (silver mirror test) is positive for aldehydes, which are oxidized to carboxylate ions while $Ag^+$ is reduced to metallic silver. Acetaldehyde ($CH_3CHO$) is an aldehyde → positive. Acetone and benzophenone are ketones (do not respond to Tollens'). Formaldehyde also gives a positive test, but acetaldehyde is listed here as the textbook answer.