The correct answer is D) Acts as backup insulation behind firebrick, plastic refractory, or castable refractory. Insulating brick serves as a secondary layer of insulation behind the primary refractory materials, such as firebrick, plastic refractory, or castable refractory. Its primary function is to provide additional thermal insulation and protect the structural integrity of the boiler or furnace casing. The insulating brick helps to maintain the high temperatures required for efficient combustion and heat transfer, while also preventing heat loss through the outer casing. The other answer choices are incorrect because they do not accurately describe the primary function of insulating brick. Option A refers to a gas-side layer, which is not the role of insulating brick. Option B describes structural stability, which is not the main purpose of insulating brick. Option C describes the first layer of the inner casing, which is not the function of insulating brick.