The correct answer is B) Space heaters should be used to prevent condensation of moisture. During periods of inactivity, large electric motors are susceptible to moisture buildup, which can lead to corrosion and insulation breakdown. Using space heaters to maintain a constant temperature around the motor helps prevent the formation of condensation, which is the recommended practice to preserve the motor's condition when not in use. The other options, such as applying varnish, using solvents, or blowing compressed air, are not the primary methods for maintaining large electric motors during periods of inactivity. These practices may be useful for general motor maintenance, but they do not directly address the issue of moisture control, which is the key consideration for inactive periods.