Which agent is rated for use on Class A fires?

The correct choice, ammonium phosphate, is an effective agent used for Class A fires, which involve ordinary combustibles such as wood, paper, cloth, and some plastics. Ammonium phosphate functions as a dry chemical fire extinguisher that interrupts the combustion process and prevents re-ignition by smothering the flames and cooling the burning materials.

In the context of fire extinguishing agents, it's important to understand the classifications of fires. Class A fires require agents that can cool and smother materials that ignite from solid fuels. Ammonium phosphate reacts chemically with the burning materials and disrupts the combustion process effectively.

In contrast, potassium chloride, potassium bicarbonate, and sodium bicarbonate are primarily used for Class B or Class C fires, which involve flammable liquids or electrical equipment respectively, and are not specifically rated for use on Class A fires.