The correct answer is A) leaving the registers open on a hot boiler. Thermal shock can occur when a hot boiler is suddenly exposed to cold air, such as by opening the boiler registers. This rapid temperature change can cause the brickwork in the boiler furnace to crack and become damaged. Leaving the registers open on a hot boiler allows the furnace to cool too quickly, leading to this thermal shock effect. The other options are incorrect because: B) load changes would not directly cause thermal shock, C) allowing the furnace to cool slowly would not lead to thermal shock, and D) the economizer is not directly related to thermal shock in the furnace brickwork.