What type of dry chemical is best suited for extinguishing fires caused by ordinary combustibles such as paper and cardboard?

The correct choice is ammonium phosphate, which is commonly used in dry chemical fire extinguishers specifically designed for Class A fires, which involve ordinary combustibles such as paper, cardboard, wood, and some plastics. Ammonium phosphate works by interrupting the chemical reaction of combustion, providing not only a suppression effect on the flame but also creating a cooling effect on the burning material.

Understanding fire classifications is crucial when selecting a fire extinguishing agent. Class A fires require agents that can effectively cool the material and prevent re-ignition, which ammonium phosphate accomplishes well due to its properties. It is a versatile agent that can also be effective on Class B fires (flammable liquids) due to its ability to create a barrier between the fuel and the fire.

For reference, sodium bicarbonate is primarily effective for Class B and Class C fires, potassium chloride, while effective in certain environments, is less commonly used for ordinary combustibles, and calcium carbonate is not typically used for extinguishing fires at all. Hence, ammonium phosphate is the most appropriate choice for combating fires involving ordinary combustible materials.