Final answer:
a) M203. The M203 grenade launcher can be attached to the M16A2E3 rifle.
Step-by-step explanation:
The grenade launcher that can be attached to the M16A2E3 rifle is the M203.
The M203 grenade launcher is a single-shot, breech-loading weapon that can fire several types of 40mm grenades, including high explosive, smoke, and illuminating rounds. It attaches to the rifle's barrel and allows the operator to engage targets at longer distances with explosive firepower.
The M320, M79, and Mk 19 are all other types of grenade launchers, but they are not compatible with the M16A2E3 rifle.