The correct answer is A) Oil is sprayed through nozzles at the point of gear mesh. This method is commonly used to lubricate the main propulsion medium-speed diesel engine reduction gears on vessels. The oil is sprayed directly onto the meshing gear teeth, providing adequate lubrication and cooling at the critical point of contact. This method ensures the gear teeth are properly lubricated, reducing wear and tear on the reduction gears. The other options are incorrect because: B) Oil rings are typically used for slower-speed gears, not medium-speed diesel engine reduction gears. C) Running the gears through an open oil sump is an older and less efficient method. D) Pressure-fed internal oil passages are more commonly used for high-speed gears, not medium-speed diesel engine reduction gears.