The correct answer is D. Fires in combustible metals, such as sodium or magnesium, are classified as Class D fires. This is because these metals are highly reactive and require specialized extinguishing agents, such as dry powder extinguishers, to effectively put out the fire. Class A, B, and C fires involve ordinary combustible materials, flammable liquids and gases, and electrical fires, respectively, which require different extinguishing methods. The other answer choices are incorrect because they do not accurately reflect the classification of fires involving combustible metals.