Which system performs the function of the Flight Director?

The function of the Flight Director is primarily performed by the Automatic Flight Control System (AFCS). This system integrates various components to provide guidance to the pilot for maintaining the desired flight path. The Flight Director displays commands on the cockpit instruments, allowing the pilot to make necessary adjustments to the control surfaces to follow the indicated trajectory.

While the Electronic Flight Instrument System (EFIS) is crucial for displaying flight data and the Multi-Function Display (MFD) can show various operational information, it is the AFCS that directly controls the flight director's functions, ensuring that the aircraft is navigated according to the flight plan and current flight parameters. The Modular Avionics Unit (MAU) supports the overall avionics architecture but does not specifically embody the behavior of the Flight Director itself.

Thus, the Automatic Flight Control System plays a key role in the operation of the Flight Director, making it the correct answer in this context.