The correct answer is A) two red lights in a vertical line. This is the correct answer because according to the International Regulations for Preventing Collisions at Sea (COLREGS), a vessel engaged in underwater operations is required to display two red lights in a vertical line to indicate the presence of an obstruction on the port side of the vessel. This helps to warn other vessels in the area of the potential hazard. The other answer choices are incorrect because a single red light (B) would not clearly indicate the presence of an obstruction, a floodlight (C) is not the proper lighting configuration, and any visible lights (D) would not specifically mark the obstruction on the port side.