The correct answer is C) To accommodate the changing length of the water drum as it expands or contracts with temperature changes. Boiler sliding feet are designed to allow for the thermal expansion and contraction of the water drum within the boiler. As the water in the drum heats up and cools down, the drum's length will change. The sliding feet provide the necessary flexibility to accommodate these length changes, preventing stress and potential damage to the boiler structure. The other options are incorrect because they do not accurately represent the primary purpose of boiler sliding feet. Option A is incorrect as the sliding feet do not create an airtight seal. Option B is incorrect as the sliding feet are not meant to compensate for hull deflection. Option D is incorrect as the sliding feet are not specifically designed to accommodate unequal expansion between the wrapper and tube sheets.