A _____ shall be used to support a wall-mounted water closet.

A concealed metal carrier is specifically designed for supporting wall-mounted water closets, ensuring that they are securely anchored to the wall. This type of carrier provides the necessary structural support to hold the weight of the toilet while also allowing for proper alignment and stability. It typically includes brackets and a frame that are concealed behind the wall, maintaining a clean aesthetic appearance in the bathroom.

The other options listed serve different functions in plumbing. A flush mount hanger is used for securing pipes to walls or ceilings but does not provide the structural support needed for a toilet. A wall flange is intended for connecting pipes and fixtures but is not designed to support the weight of a water closet. A horn adapter is typically associated with connecting different plumbing components rather than providing support for fixtures like toilets. Thus, the concealed metal carrier is the appropriate choice for this application.