What power source does the middle PFD display represent?

The correct answer indicates that the middle Primary Flight Display (PFD) is powered by the main bus. The main bus is a critical component in an aircraft's electrical system, supplying power to essential systems, including the PFDs. The PFDs are vital for presenting flight information to the pilots, and ensuring that the middle PFD is connected to the main bus indicates that it is receiving a robust and reliable power source, which is essential for safe operation.

In flight operations, especially under Part 135, having a stable and consistent power supply for navigational and operational displays significantly contributes to overall situational awareness and safety. Thus, recognizing the middle PFD's connection to the main bus emphasizes its importance in the aircraft's electrical architecture.