The correct answer is A) Class "A" fire. A Class "A" fire is a fire involving ordinary combustible materials such as wood, paper, cloth, rubber, and many plastics. A burning mattress would be considered a Class "A" fire because it is composed of these types of ordinary combustible materials. The other answer choices are incorrect because: B) Class "B" fires involve flammable liquids and gases, C) Class "C" fires involve energized electrical equipment, and D) Class "D" fires involve combustible metals, none of which accurately describe a burning mattress fire.