Which handle prevents smoke and fumes from entering the cockpit after an engine shutdown?

The correct option, which is the firewall shutoff, is designed specifically to prevent smoke and fumes from entering the cockpit after an engine shutdown. This is a critical safety feature in aircraft design. When activated, the firewall shutoff closes off fuel supply lines or other pathways that could allow hazardous fumes from the engine compartment to infiltrate the cockpit area.

This mechanism enhances the safety of both the crew and passengers by ensuring that any potential contaminants or hazardous conditions are isolated from the cabin environment, especially in emergency situations. The maintenance of a clean air supply in the cockpit is essential for ensuring pilot visibility and performance during critical phases of flight or emergencies.

The other handles and controls mentioned would not serve the same preventative function regarding smoke and fumes. For instance, the throttle control manages the engine power output, while the fire extinguisher handle is used in case of an in-flight fire, and the cabin air control primarily influences the internal air circulation or temperature within the cabin itself, not preventing external smoke or fumes from entering.