The correct answer is A) To ensure that the valve will not be seized shut when hot. The main steam stop valve of an auxiliary boiler should be eased off its seat and then gently closed before lighting off to ensure that the valve will not become seized shut when the boiler heats up. This is a crucial step to prevent the valve from becoming stuck in the closed position, which could lead to dangerous pressure buildup in the boiler. The other answer choices are incorrect because they do not address the primary reason for this procedure, which is to prevent the valve from seizing due to thermal expansion.