What is the role of the seat belt sign in signaling for MCD opening?

The seat belt sign plays a critical role in signaling that the aircraft is secure for opening the MCD (Main Cabin Door). When the seat belt sign is turned off, it indicates that it is safe for passengers to move about the cabin, including exiting the aircraft. Prior to opening the MCD, the flight crew must confirm that it is safe to do so, including ensuring that the aircraft is on the ground, parking brakes are set, and engines are shut down.

Once the necessary checks are completed and the seat belt sign is off, it communicates to the cabin crew that it is now safe to proceed with opening the doors. This procedure helps avoid any potential hazards that could arise if doors were opened while the aircraft is still in transition or if passengers are still expected to be seated.

In contrast, the other options do not accurately represent the main function of the seat belt sign in relation to the MCD. The sign doesn't directly indicate whether boarding is allowed or complete, nor does it specifically mandate that passengers remain seated during landing or taxiing. Instead, it is fundamentally about ensuring safety when the aircraft is ready for cabin movement and door operation.