What must be done to a locking door between compartments?

A locking door between compartments in an aircraft is subject to specific regulations to ensure safety and security. The correct answer indicates that the door must be able to lock with the pilot's permission. This requirement is crucial because it allows the pilot to maintain control over the accessibility of different areas within the aircraft during flight.

The ability to lock a door enhances security by preventing unauthorized access to sensitive areas, which could include the cockpit or compartments containing safety equipment. However, it is equally important that the pilot has the authority to control this locking mechanism, enabling them to manage access based on situational needs, such as ensuring cabin safety during turbulence or securing confidential areas.

Other options may not prioritize the critical aspect of pilot control and safety that is inherent to aircraft operations. For instance, having the door remain unlocked does not consider scenarios where security needs to be tightened. Similarly, it would be impractical for the door to be secured at all times if access is needed for operational reasons, and ensuring convenience for passengers alone does not align with the necessity for safety and control in aviation environments.