The rear cockpit avionics bus is powered by which unit?

The correct answer is that the rear cockpit avionics bus is powered by the generator bus. The generator bus is responsible for distributing power generated by the aircraft's generators to various systems, including avionics, which are critical for navigation, communication, and monitoring system functionality. This ensures that avionics equipment, located in the rear cockpit, receives the necessary power directly from the airplane's generators, supporting operational readiness during flight.

The other options refer to different power distribution systems that serve varying functions in an aircraft. The battery bus, for example, typically powers essential systems when the aircraft is not running or when generator power is not available, but it is not the primary source of power for avionics during normal operation. The power bus generally describes any bus that transmits power but isn't specific to avionics, while the control bus relates more to data transmission between systems rather than actual power supply. Each of these options serves a distinct purpose, but in the case of powering the rear cockpit avionics bus, the generator bus is the correct source.