What type of maintenance occurs primarily at the D-level?

D-level maintenance, or depot-level maintenance, is the most intensive type of maintenance performed on aviation assets. It is typically carried out at a facility that specializes in major repairs and overhauls. The emphasis is on comprehensive rebuilding, repairing, and overhauling of aircraft and components, which directly aligns with the principles of rework maintenance.

Rework maintenance focuses on the restoration of capabilities that have diminished due to usage and time. This often includes extensive inspections, upgrades, and modifications. Since D-level facilities have the resources, expertise, and infrastructure to conduct this significant type of maintenance, it is considered the appropriate match for the characteristics of rework maintenance.

The other options do not typically occur primarily at the D-level. Acceptance upkeep and transfer upkeep are associated with the initial acceptance of new systems or the transfer of equipment between organizations, whereas phase upkeep usually involves routine checks and minor repairs that are more characteristic of I-level (intermediate) or O-level (organisational) maintenance functions. Thus, rework maintenance stands out as the correct answer for D-level maintenance.