The correct answer is D) 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. Draining the condenser of water is the appropriate action, as leaving water in the system can lead to rusting and scaling of the internal surfaces. This is in accordance with standard industry practices and regulations for preserving a ship's equipment during a prolonged layup. The other options are incorrect because: A) filling with moist air would not provide adequate protection, B) pressurizing with nitrogen is unnecessary for a condenser, and C) leaving the condenser under a vacuum could potentially cause damage to the equipment.