The correct answer is D) Booster pump discharge. The fuel oil back pressure regulator is designed to maintain a constant pressure in the fuel supply line by returning excess fuel back to the booster pump discharge. This ensures a consistent fuel flow to the engine, which is crucial for proper engine operation. The other answer choices are incorrect because: A) the booster pump suction would not be the correct return point, as that would disrupt the pump's operation; B) the purge valve discharge is not the intended return path for the back pressure regulator; and C) the fuel oil day tank is not the appropriate destination for the excess fuel.