A unit reports its readiness to higher headquarters using which of the following reports?

The Status of Requirements and Training Support System (SORTS) is the correct choice because it is the designated reporting system that units utilize to communicate their readiness status to higher echelons. SORTS provides a snapshot of a unit's overall readiness, including personnel, equipment, and training proficiency, thereby offering critical information for decision-makers regarding personnel and resource allocation.

SORTS is specifically designed to capture and report on the essential readiness parameters that indicate how well a unit can perform its assigned tasks and missions. This standardized system allows for consistent reporting across different units and services, which is crucial for effective command and control.

The other options do not serve the same purpose as SORTS. The Status of Training and Operations Report focuses more on training progress and operational readiness but does not encompass the full breadth of requirements like SORTS. The Unit Readiness Report may provide some readiness information, yet it is not the standard used for official reporting in the same context as SORTS. The Mission Readiness Assessment evaluates a unit's performance in specific missions but lacks the comprehensive nature of the SORTS reporting structure, which is more holistic and standardized across the military.