For which class of fires is foam used as a primary extinguishing agent?

Foam is primarily used as an extinguishing agent for Class B fires, which involve flammable liquids such as gasoline, oil, and solvents. The unique properties of foam create a barrier that smothers the flames and separates the fuel from the oxygen, effectively stopping the combustion process. This is crucial in controlling and extinguishing Class B fires, as it reduces the chances of reignition by covering the flammable surface with a foam blanket.

In contrast, Class A fires, which involve ordinary combustibles like wood and paper, typically require water or other extinguishing methods that cool the material. Class C fires involve energized electrical equipment and require agents that do not conduct electricity, such as carbon dioxide or dry chemical extinguishers. Class D fires are related to combustible metals and require specialized agents that can effectively handle such materials. Thus, foam's effectiveness in forming a film on top of flammable liquids makes it the most appropriate choice for combating Class B fires.