During preflight, what action must be taken for the external CFS handle doors?

The correct choice emphasizes the critical safety protocol regarding the external CFS (Cargo Fire Suppression) handle doors during preflight inspections. Ensuring that these doors are closed and locked is vital as it prevents any accidental activation or compromise of the fire suppression system during flight. Keeping the doors secure guarantees that the system will function correctly in the event of an emergency, thereby protecting both the aircraft and its occupants.

This action is crucial for maintaining operational integrity and aligns with established safety regulations that dictate the condition of equipment before takeoff. By locking the doors, the crew ensures that unauthorized access is prevented and that the cargo compartment is sealed properly, thereby minimizing risks associated with ventilation or potential hazards due to improperly secured equipment.