Which weapon is designated as the M240B?

The M240B is indeed categorized as a medium machine gun, which makes this choice the correct answer. The M240B is a belt-fed, gas-operated machine gun that is designed for sustained fire support. It is mounted on vehicles, tripods, and can also be used as a light machine gun by infantry soldiers. This versatility allows it to fulfill various roles on the battlefield, providing effective suppressive fire against enemy positions.

Other weapons mentioned serve different purposes. The M249 is a light machine gun that provides a squad-level support role, whereas the M2 Browning is a heavy machine gun typically used for anti-aircraft and anti-vehicle roles due to its larger caliber and power. The M870 is a shotgun, used primarily in close-quarters combat situations or for specific missions requiring breaching capability. Each of these choices serves distinct functions within military operations, highlighting the M240B's unique position as a medium machine gun within the infantry arsenal.