The correct answer is D) Constant tension mooring winch system. The installation of an oil cooler is necessary in hydraulic systems that generate significant heat, such as the constant tension mooring winch system. This is because the high-power demands of the mooring winch can lead to the hydraulic fluid overheating, which can damage the system components. An oil cooler helps dissipate this heat and maintain the proper operating temperature of the hydraulic fluid. In contrast, the hatch cover system, watertight door system, and internal combustion engine hydraulic starter system typically do not generate as much heat, so an oil cooler may not be necessary in these applications.