The correct answer is B) leaving the registers open on a hot boiler. Thermal shock can cause fracturing and damage to boiler furnace brickwork. Leaving the registers (openings) on a hot boiler open allows cold air to rapidly enter the furnace, leading to sudden temperature changes that create thermal shock and damage the brickwork. This is because the cold air cools the hot brickwork too quickly, causing it to crack and break. The other options are incorrect because: A) allowing the furnace to cool slowly avoids thermal shock, C) load changes are less likely to cause the rapid temperature changes that lead to thermal shock, and D) the economizer is designed to gradually preheat the feed water to avoid thermal shock.