The correct answer is C) breaking the chain reaction. Dry chemical extinguishers, such as those containing sodium bicarbonate or potassium bicarbonate, extinguish Class B fires (flammable liquids and gases) primarily by breaking the chemical chain reaction that sustains the fire. The dry chemical powder interferes with the combustion process, interrupting the self-sustaining cycle of fuel, heat, and oxygen that feeds the fire. This disruption of the fire's chemical chain reaction is the most effective mechanism for dry chemical extinguishers against Class B fires. The other options are incorrect because they do not accurately describe the primary extinguishing mechanism of dry chemicals. Smothering and oxygen dilution are more characteristic of other extinguisher types, while cooling is not the primary means by which dry chemicals suppress Class B fires.