The correct answer is D) a Butterworth system using crude oil instead of water as the cleaning medium. A Crude Oil Wash (COW) system is a cleaning system used on tankers to clean the cargo tanks. It uses the cargo crude oil itself, instead of water, as the cleaning medium. This is done by circulating the crude oil under pressure through a series of permanently installed Butterworth cleaning machines within the cargo tanks, similar to a water-operated Butterworth system (option C). This makes option D the correct choice, as it accurately describes the COW system as a Butterworth system that uses crude oil instead of water. The other options are incorrect because they do not accurately describe the function of a COW system. Option A refers to cleaning pump room bilges, which is not the purpose of a COW system. Option B describes a mechanism for filtering and purifying crude oil, which is different from the tank cleaning function of a COW system.