Which round does not produce illumination?

The correct choice identifies the M889 round as the one that does not produce illumination. The M889 is specifically designed for practice and training purposes, meaning it simulates the effects of a live round without the illumination capabilities found in other rounds used for signaling or marking.

Other rounds mentioned, such as the M816 and M880, are designed for illumination purposes. The M816 is an illuminating round that expels a bright flare upon detonation to aid in nighttime operations. Similarly, the M880 also serves a role in illumination. The M821/A1 is primarily a high-explosive round, but it can also potentially be used in conjunction with devices that require illumination effects.

Identifying the M889 as strictly a practice round emphasizes its non-combat role, focusing on training scenarios without the additional effects of illumination, which distinguishes it from the other options provided. This understanding is crucial for effective ammunition selection during training exercises where illumination is not necessary or desired.