The correct answer is C) control steam admission and maintain the proper steam spray pattern in the DC heater. The steam control valves installed in the auxiliary exhaust line are used to regulate the flow of steam into the direct contact (DC) heater, which is a type of feedwater heater. The purpose of these valves is to control the steam admission and maintain the proper steam spray pattern within the DC heater. This ensures that the steam is effectively utilized to heat the feedwater, which is a critical function for the proper operation of the steam plant. The other answer choices are incorrect because they do not accurately describe the purpose of the steam control valves in the auxiliary exhaust line. Option A is incorrect as the valves do not regulate back pressure in the desuperheater line. Option B is incorrect as the valves do not seal the vent condenser. Option D is incorrect as the valves do not preheat the condensate before it enters the vent condenser.