To which weapon is the M203 designed to be attached?

The M203 is a single-shot 40mm grenade launcher that is specifically designed to be attached to the M16 or M4 series of rifles. This attachment allows the operator to fire grenades while maintaining the primary function of a rifle, providing versatility in combat situations. The integration of the M203 with the M16/M4 platform enhances the weapon's capability by allowing the user to deliver explosive ordinance without needing a separate launcher. This design is significant for infantry operations, as it combines the functions of small arms and indirect fire support in a single system.

While other weapons mentioned may serve various roles in military operations, they are not designed to be used in conjunction with the M203 in the same way that the M16/M4 platforms are. The M249, for instance, is a light machine gun and operates independently of the grenade launcher, while the MP5 is a submachine gun that lacks compatibility with the M203 mounting design. SOCOM, referring to U.S. Special Operations Command, involves various weapons systems but does not denote a specific platform designed for the M203. Thus, the M16/M4 is the correct answer as it directly supports the operational use of the M203 grenade launcher.