Which classification of fire extinguisher is used for materials that can create a hazardous chemical reaction when wet?

The classification of fire extinguisher primarily used for materials that can create a hazardous chemical reaction when wet is indeed the one classified as D. This category specifically covers fires involving reactive metals such as sodium, potassium, magnesium, and others that can combust or produce hazardous reactions in the presence of water. The extinguishing agents used for Class D fires are typically dry powders that can smother and cool the burning metal without reacting with it.

In terms of other classes of fire extinguishers: Class A is used for ordinary combustibles like wood and paper, Class B is for flammable liquids, and Class C is suitable for electrical fires. Each of these classifications addresses specific types of fire sources but does not pertain to the unique hazards posed by reactive metals, hence making them unsuitable for handling fires involving materials that react adversely when wet.