The correct answer is D) refractory. The brickwork surrounding the firebox of a boiler is known as the refractory. The refractory is the heat-resistant lining that protects the boiler's structure from the high temperatures of the fire. It is made of specialized materials that can withstand the intense heat and prevent heat loss from the firebox. The other answer choices are incorrect because they refer to different components of the boiler system. The waterwall is the tube assembly that surrounds the firebox and absorbs heat, the fire plate is the steel plate that forms the front of the firebox, and the screen wall is a barrier that helps control air flow in the boiler.