The correct answer is B) prevent the formation of a vacuum within the boiler. When a boiler is taken offline and begins to cool, the steam condenses, which can create a vacuum inside the boiler. Opening the air cock allows air to enter the boiler, preventing this vacuum from forming. This is important to prevent potential damage to the boiler from the vacuum. The other answer choices are incorrect because: A) is not necessary, as the steam drum will naturally purge air as it cools; C) is not relevant, as the superheater is not involved in this scenario; and D) is not the primary purpose of opening the air cock, which is to prevent vacuum formation, not necessarily to cool the steam drum evenly.