The correct answer is C) completely drained of water. When a ship is to be laid up for an indefinite period, it is important to properly preserve the main condenser to prevent corrosion and damage. Completely draining the water from the steam side of the main condenser is the recommended practice, as this removes the source of potential corrosion and ensures the internal surfaces are exposed to dry air rather than moist conditions. The other options are not the correct approach for long-term lay-up. Leaving the condenser under a vacuum (A) can lead to air leaks and the introduction of moisture, while filling it with moist air (B) would promote corrosion. Pressurizing the condenser with nitrogen (D) is generally not necessary for extended lay-up and could potentially cause damage if not properly executed.