What does the military prefix 'R' stand for?

The military prefix 'R' stands for the Army National Guard. In the context of military terminology, prefixes are used to designate specific branches or components of the armed forces. The Army National Guard is a reserve component of the United States Army, and the use of the 'R' prefix signifies its status as part of the National Guard. It is important to distinguish this from other military branches, as each has its own unique prefixes to avoid confusion in communication and identification. The other options represent different branches or components and are denoted by different prefixes, which are not applicable in this context.