The correct answer is C) Screen tubes. In a water tube marine boiler, the screen tubes act as a protective barrier between the superheater tubes and the direct heat of the combustion fires. The screen tubes are positioned between the superheater and the furnace, shielding the superheater tubes from the intense heat of the flames. This design feature helps prevent damage to the superheater tubes, which are critical for the boiler's efficient operation. The other options are incorrect because they do not directly protect the superheater tubes from the combustion fires. Downcomers (A) and water drums (D) are part of the water circulation system, while water-wall tubes (B) line the furnace walls but do not specifically shield the superheater.