The correct answer is C) force the feed water into the boiler. The pressure in the feed water system must exceed the boiler steam drum pressure to force the feed water into the boiler. This higher pressure overcomes the back pressure in the steam drum, allowing the feed water to enter the boiler. Option A is incorrect because air leakage is prevented by maintaining a positive pressure in the feed water system, not by exceeding the steam drum pressure. Option B is incorrect because water hammer is prevented by other design features, not by exceeding the steam drum pressure. Option D is incorrect because the higher feed water pressure is used to force the water into the boiler, not to remove steam from the steam drum.