The correct answer is C) peeling or spalling of the brickwork. Slag buildup on boiler furnace refractory is undesirable because it causes the refractory materials to expand and contract at different rates, leading to peeling or spalling (chipping/flaking) of the brickwork. This happens as the slag accumulates and insulates parts of the refractory, causing uneven heating and thermal stresses that damage the brickwork. The other answer choices, such as shrinking, excessive cooling, or fracturing of anchor bolts, are not the primary issues caused by slag buildup on the furnace refractory.