The correct answer is D) vertical U-type. The vertical U-type superheater is characterized by the superheater tubes being arranged in a U-shape and oriented vertically within the boiler. This configuration is commonly used in Coast Guard-inspected vessels. The vertical orientation and U-shape allow for efficient heat transfer from the boiler's hot gases to the superheater tubes, which in turn superheats the steam. The other options are incorrect because: A) a horizontal U-type superheater has the tubes arranged in a U-shape but oriented horizontally; B) an overdeck convection-type superheater is located above the boiler deck; and C) an overdeck integral-type superheater is also located above the boiler deck, integrated with the boiler design.