If a passenger stands up while the FD door is being opened, what must happen?

The correct answer is that the door remains closed until the passenger is seated because passenger safety is the top priority in aviation. The procedure is designed to prevent any potential risks associated with opening the door while passengers are standing. An open door with passengers standing nearby creates a dangerous situation, as it increases the risk of an accidental fall, exposure to the outside elements, or disruption during the boarding process.

By ensuring that the door remains closed until all passengers are seated, the cabin crew can maintain order and enhance safety, giving time for the necessary checks to be made before the boarding process continues. This protocol is part of standard operating procedures that help ensure a smooth and secure boarding process in an aircraft environment.

Other choices do not appropriately focus on the safety measures that are vital to maintaining order during the boarding or deboarding process.