The correct answer is C) Increased impeller and wear ring clearance. As the main salt water cooling pump ages, the clearance between the impeller and the wear ring can increase due to normal wear and tear. This increased clearance allows more water to bypass the impeller, reducing the discharge pressure of the pump. This is the most likely cause of the decreased discharge pressure over the three-year period since the last overhaul. The other options are less likely explanations. A lower bus frequency would not directly affect the pump's discharge pressure. Worn pump shaft bearings could also reduce pressure, but this is less likely than increased clearance over time. A fouled suction strainer would increase the pump's suction side pressure drop, not the discharge pressure.