The correct answer is D. A magnesium fire is classified as a Class D fire. Class D fires involve combustible metals, such as magnesium, sodium, and potassium. These types of fires require specialized extinguishing agents and methods, as water and other common extinguishers can actually make these fires worse. Class D fire extinguishers, which use dry powder or other agents specific to metal fires, are required to safely extinguish a magnesium fire. The other answer choices are incorrect because: Class A fires involve ordinary combustible materials, Class B fires involve flammable liquids and gases, and Class C fires involve energized electrical equipment.