For what purpose is the harness D-ring located at the back of the MK20 used?

The harness D-ring located at the back of the MK20 is primarily used for lifting purposes. This D-ring provides a secure point for attaching lift bags, other equipment, or personnel in a way that ensures safety when divers are being lifted from or lowered into the water. It is designed to withstand significant loads and is integral to safety protocols surrounding diving operations, particularly during recovery. Proper utilization of this D-ring enhances the efficiency and safety of diving operations, ensuring that divers can be safely brought back to the surface or positioned for deployment.

The other functions suggested by the incorrect choices may have some relevance in different contexts, but the primary and most critical use of the D-ring in this operational setting is specifically designated for lifting, ensuring the diver's safety during retrieval or deployment.