________ powers the DC SERV BUS.

The DC SERV BUS in an aircraft is powered by both the main battery and the APU, which is why the selected answer encompasses both power sources. The main battery provides essential electrical power for the aircraft systems, particularly when the engines are not running. The Auxiliary Power Unit (APU) acts as a backup generator, providing electrical power when the engines are not operating, particularly during ground operations.

In many aircraft electrical systems, the integration of these two power sources ensures redundancy and reliability. When the APU is running, it can supply power to the DC SERV BUS alongside the main battery. This design is critical for maintaining the aircraft's essential systems operational, especially when the aircraft is on the ground or during emergency situations.

While the inverter converts DC power to AC, it is not directly responsible for powering the DC SERV BUS. Therefore, the best answer must reflect the roles of both the main battery and the APU in providing power, making the selection of both sources the most comprehensive and accurate choice.