The correct answer is B) peeling or spalling of the brickwork. Slag buildup on boiler furnace refractory is undesirable because it can cause the brickwork or refractory material to peel or spall. Slag, which is a hard, glassy deposit that forms on the furnace walls, can create uneven heating and thermal expansion within the refractory. This can lead to the brickwork or refractory material cracking, flaking, or breaking apart, a process known as spalling. This is an undesirable outcome as it can compromise the integrity and performance of the boiler furnace. The other answer choices are incorrect because they do not directly address the primary issue caused by slag buildup. Fracturing of the anchor bolts, excessive cooling of the brickwork, and shrinking of the brickwork are not the main consequences of slag buildup on the furnace refractory.