The correct answer is D) A malfunctioning auxiliary exhaust make-up steam regulating valve. The lifting of the DC heater relief valve indicates that there is excessive pressure in the DC heater, which is typically caused by a malfunctioning auxiliary exhaust make-up steam regulating valve. This valve is responsible for controlling the amount of steam entering the DC heater to maintain the proper pressure and temperature. If this valve is not functioning correctly, it can allow too much steam to enter the DC heater, leading to the relief valve lifting to release the excess pressure. The other options are incorrect because they do not directly cause the lifting of the DC heater relief valve. Low water level, excessive deaeration, and low back pressure in the auxiliary exhaust line may contribute to issues with the DC heater, but they do not directly result in the relief valve lifting.