The correct answer is D) Increased capability of removing and controlling dissolved oxygen. In a closed feed water system for a marine boiler, the feed water is circulated in a closed loop, which allows for more effective removal and control of dissolved oxygen. This is important because dissolved oxygen can lead to corrosion and scale formation in the boiler, reducing its efficiency and lifespan. The closed system design enables the use of deaerators or other oxygen removal techniques, which is not as easily achieved in an open feed water system. The other options are incorrect because they do not directly address the advantage of a closed feed water system in terms of dissolved oxygen control. Option A is incorrect as a closed system does not necessarily reduce the requirement for condensate purity. Option B is incorrect as the operating pressure of the feed pump is not directly related to the system being closed or open. Option C is incorrect as the steam requirement for feed water heating is not necessarily reduced in a closed system compared to an open system.