The correct answer is D) energize the space heater circuit. When an alternator is to remain idle for even a few days, it is important to energize the space heater circuit. This helps to prevent condensation from forming inside the alternator, which can lead to corrosion and damage. The space heater helps to maintain a constant temperature within the alternator, reducing the risk of moisture buildup. The other options are not the correct approach. Lifting the brushes and disconnecting the pigtails (A) is unnecessary. Opening the equalizing bus disconnect switch (B) does not address the issue of condensation. Insulating the collector rings with cardboard (C) would not effectively prevent moisture from accumulating.