Which of the following calibers does the M16 rifle use?

The M16 rifle is designed to use the 5.56x45mm NATO caliber, which is a standard round for this weapon system. This caliber was specifically developed to provide a balance between high velocity and low recoil, allowing for improved accuracy and control during sustained fire. The 5.56x45mm NATO cartridge is known for its lightweight design and effectiveness in various combat situations, contributing to the M16's versatility and widespread use by military forces around the world.

In contrast, other calibers listed serve different firearm platforms. The 7.62x51mm NATO is used in larger rifles and machine guns, delivering more power but with increased recoil. The .45 ACP is a larger caliber typically used in handguns, known for stopping power but not suited for the M16's design. The 9mm Parabellum is primarily associated with submachine guns and pistols, making it incompatible with the M16.