Which type of fire extinguisher uses foam as the extinguishing agent?

The correct response highlights the specific role that foam extinguishers play in firefighting, particularly in relation to the types of flammable materials they are designed to combat. A Class B foam extinguisher is intended for fires involving flammable liquids, such as gasoline, oils, and paints. Foam acts as an effective extinguishing agent in these situations by smothering the fire and preventing oxygen from reaching the flames, as well as cooling the burning liquid.

Foam is advantageous because it not only suppresses flames but also forms a barrier on the surface of the liquid, preventing the release of flammable vapors, which helps to stop the fire from reigniting. This specific function is what distinguishes Class B foam extinguishers from others designed for Classes A, C, or D, which pertain to different types of fires involving ordinary combustibles, electrical equipment, and metals, respectively.