Which command should be executed to secure the aircraft for departure?

The correct command to secure the aircraft for departure is to close external doors and retract the airstair. This action is crucial because ensuring that all external doors are properly closed prevents any potential hazards during taxiing, takeoff, or flight. It also signifies to other crew members and ground staff that the aircraft is ready for departure, aligning with safety protocols. The retraction of the airstair is equally important, as it ensures that no ground equipment is in the way of the aircraft's movement, further facilitating a safe departure process.

The other options, while important in different contexts of aircraft operations, do not directly relate to the immediate action of securing an aircraft for departure. For instance, commands to activate an airstart unit or request a wingwalker serve different purposes within the pre-departure checklist and are not directly involved in the steps needed to secure the aircraft prior to taking off.