During flight in RVSM airspace, what must the ADC source coupled to the active autopilot be the same as?

In the context of RVSM (Reduced Vertical Separation Minimum) operations, aircraft are required to maintain a very precise altitude to ensure safe separation from other aircraft. This precision is critical, as RVSM airspace allows for a reduced vertical separation of only 1,000 feet between aircraft flying between altitudes of 29,000 feet and 41,000 feet.

For the autopilot to function correctly during flight in RVSM airspace, the Air Data Computer (ADC) source that is providing altitude information must be the same source that feeds the active autopilot. This ensures that altitude data used for autopilot functions and altitude reporting to ATC are consistent, which is essential for maintaining correct altitude levels and adhering to the RVSM requirements.

The ATC transponder, which is responsible for providing altitude reports to air traffic control, must be functioning with the same ADC source utilized by the autopilot system. This ensures that both the autopilot and transponder are operating with the same altitude reference, promoting safety and communication accuracy regarding altitude among surrounding aircraft.

While the other options may relate to the overall systems in the aircraft, they do not directly pertain to the specific requirement that the ADC source for the autopilot must match with the transponder to ensure