What type of portable fire extinguisher mechanism uses compressed air or inert gas to expel the agent?

The correct answer is the type of extinguisher that uses compressed air or an inert gas to expel the extinguishing agent, which is the stored pressure mechanism. This type of extinguisher is designed to have the extinguishing agent and the propellant stored in the same container. When the nozzle is activated, the pressure within the cylinder forces the agent out, enabling the firefighter to direct it effectively onto the fire.

Stored pressure extinguishers are advantageous because they are ready for immediate use, as they maintain pressure over time without requiring additional action to pressurize the system just before use. They are commonly seen in many fire protection settings due to their efficacy in various firefighting scenarios.

While carbon dioxide extinguishers also utilize gas to expel their content, they specifically use carbon dioxide as the extinguishing agent itself rather than an inert gas to assist in the expulsion. Pump action extinguishers rely on a manual pumping mechanism to create pressure, and aqueous film-forming foam extinguishers involve a different technological approach with foam generation, which does not incorporate the principle of stored pressure in the same manner.