What command is given as a sign to begin exercising?

The command given as a sign to begin exercising is "Forwards, march!" This command typically signals the start of physical training or the start of a movement in marching formation. It indicates that personnel should begin their exercises in an orderly and synchronized manner.

In the context of military training, commands are very specific and serve distinct purposes. "Forwards, march!" directly implies that individuals should start moving, which is essential in physical fitness routines often conducted as a group in a controlled environment. This command fosters discipline and ensures that everyone begins their exercises together.

Other choices may refer to different contexts. "Attention!" is used to bring troops to a formal position, while "Fall in!" directs individuals to assemble in formation. "Ready, set, go!" is commonly associated with races or informal physical activities rather than military exercises, where pinpoints and formations are key. Hence, "Forwards, march!" is suitably aligned as the command that initiates organized exercising in a military context.