The correct answer is A) distributes feed water evenly throughout the steam drum. The internal feed pipe in a D-type marine boiler is designed to ensure the even distribution of feed water throughout the steam drum. This even distribution helps maintain a consistent water level and temperature in the boiler, which is essential for efficient and safe boiler operation. The other answer choices are incorrect because: B) relates to the distribution of boiler feed water chemicals, not the physical distribution of the feed water itself; C) describes the location of the deaerator, not the feed pipe; and D) refers to the downcomer tubes, which are a separate component from the internal feed pipe.