The correct answer is C) Check valve. A check valve is a type of valve that allows fluid (in this case, water) to flow in only one direction, preventing backflow. In the scenario of a flooded deaerating feed tank, a check valve in the auxiliary exhaust line would prevent water from flowing back into the line, which could otherwise occur due to the hydrostatic pressure of the flooded tank. The check valve acts as a one-way barrier, ensuring that water does not contaminate the exhaust system. The other options are incorrect because: A) A reverse-acting relief valve is designed to relieve excess pressure, not prevent backflow; B) Exhaust piping alone does not have a mechanism to stop water from flowing back; and D) Pumps are used to move fluid, not specifically prevent backflow.