The correct answer is B) refractory and insulation. Downcomers on auxiliary package boilers are protected from direct contact with hot gases by a layer of refractory material and insulation. This insulation and refractory lining helps prevent the hot combustion gases from directly impinging on the downcomers, which could potentially damage them. The other answer choices, such as steel baffles, water wall tubes, or screen tubes, are not the primary means of protecting the downcomers from the hot gases in this type of boiler system.