Which classes of fires can foam fire extinguishers effectively combat?

Foam fire extinguishers are particularly effective against Class A and Class B fires. Class A fires typically involve ordinary combustible materials such as wood, paper, and textiles, while Class B fires involve flammable liquids such as gasoline, oil, or solvents. The design of foam extinguishers allows them to smother and cool the burning material, which makes them suitable for both classes, but especially in situations where flammable liquids are present.

Class C fires, which involve energized electrical equipment, and Class D fires, which involve combustible metals, require specialized types of extinguishing agents due to the unique characteristics of the materials burning. Using foam on electrical fires can pose a risk of electric shock or further reactions, while foam is not suitable for combating metal fires due to the reactivity of metals with water or foaming agents.

Thus, the combination of Class A and Class B fires makes Class AB fires the correct context for the application of foam fire extinguishers, establishing why the response indicating Class AB fires is valid in the context of this question.