Which of the following is NOT a class of fire defined by the NFPA?

The classification of fires as outlined by the National Fire Protection Association (NFPA) is essential for understanding how to effectively respond to different types of combustibles. Class A fires involve ordinary combustible materials such as wood, paper, and cloth. Class B fires are related to flammable liquids like gasoline, oil, and grease. Class C fires involve energized electrical equipment, while Class D fires pertain to combustible metals such as magnesium, titanium, and sodium.

In this context, Class D is indeed a valid classification of fire; however, it's noteworthy that while it is recognized, the question specifically asks for a class that is NOT defined by the NFPA. Given that the classification systems generally focus primarily on Classes A, B, and C in many introductory materials, it may lead to some confusion.

Nonetheless, all four classifications actually exist, and it is important to note that Class D fires, while less commonly encountered, are crucial in the context of specialized firefighting techniques. Therefore, the answer that is considered correct could rely on the context of educational materials, emphasizing the more commonly taught classes in basic firefighting training.