The correct answer is D) Reduce the probability of corrosion. When a propulsion boiler is removed from service for an extended period, the firesides should be thoroughly dried after water washing to prevent corrosion. Leaving the firesides wet can lead to the formation of rust and scale, which can damage the boiler's metal surfaces over time. Drying the firesides helps remove any residual moisture and reduces the likelihood of corrosion occurring during the boiler's downtime. The other options are incorrect because they do not directly address the main reason for drying the firesides. Flarebacks on lighting off (A) and thermal spalling (B) are more related to the boiler's operation, not its long-term storage. Cracking of the brickwork (C) is not the primary concern when a boiler is taken out of service.