The correct answer is B) After the main unit is secured, lubricating oil should be circulated until the temperature of the oil and reduction gear casing approximates the engine room temperature. This is the correct answer because allowing the reduction gear casing to cool down to the engine room temperature after the main unit is secured helps prevent the accumulation of condensate. When the hot reduction gear casing cools down rapidly, it can cause condensation to form, which can lead to corrosion and damage. Circulating the lubricating oil until the temperatures equalize helps maintain a more gradual cooling process and avoid this issue. The other options are incorrect because they do not directly address the issue of condensate accumulation in the main reduction gear casing. Option A is about maintaining the proper lubricating oil temperature during operation, option C is about the use of gland sealing steam, and option D is about maintaining a specific lubricating oil pressure, none of which are directly relevant to preventing condensate buildup.