Which of the following is NOT a type of ammunition used with the M16/A2 Rifle?

The M204 round is not a type of ammunition used with the M16/A2 Rifle, which is why it is the correct answer. The M16/A2 typically uses several standardized types of ammunition, including the M855, which is a standard ball round designed for effective range and penetration, M199 dummy rounds used for training purposes, and the M200 blank rounds used for training exercises where live ammunition is not appropriate and simulate firing without a projectile.

In contrast, the M204 designation for tracer rounds does not exist in the standard nomenclature associated with the M16 series. Tracer ammunition is typically designated differently and would not fall under the M204 label. Understanding the designations and purposes of various ammunition types is crucial for effective training and operational readiness with the M16/A2 rifle.