What display provides the status of the autopilot system?

The Primary Flight Display (PFD) is the correct choice because it is specifically designed to present critical flight information to the pilots, including details regarding the autopilot system's status. The PFD displays key parameters such as altitude, airspeed, vertical speed, and heading information, along with the mode of the autopilot when it is engaged. This visual representation allows pilots to quickly assess the current operating mode of the autopilot, including whether it is armed, engaged, or in a specific operational mode such as altitude hold or flight level change.

The Navigation Display (ND) primarily focuses on route and navigation information, while the Flight Control Unit (FCU) is used for inputting commands related to the autopilot and managing its settings, but does not display the status itself. The Multi-function Control Display Unit (MCDU) serves a different purpose, providing access to the flight management system and allowing pilots to input flight plan information, but it does not present autopilot status directly on its interface. Therefore, the PFD is essential for flight monitoring and situational awareness, especially with regard to the autopilot's operation.