Which class of fire extinguishers is appropriate for fires caused by grease or fatty substances?

The appropriate class of fire extinguishers for fires caused by grease or fatty substances is Class K. Class K extinguishers are specifically designed to combat fires that originate in kitchens or areas where cooking oils and fats are present. This class uses special extinguishing agents that can effectively cool and suppress the flames of these high-temperature fires, which are different from those fueled by common combustibles or flammable liquids.

In the context of other classes, Class A extinguishers are suitable for fires involving ordinary combustibles, such as wood and paper. Class B extinguishers are designed for flammable liquids like gasoline and oil, while Class C extinguishers are intended for electrical fires. Each of these classes targets different types of fuels and would not be effective against cooking grease fires, which is why Class K is the only appropriate choice in this scenario.