The correct answer is C) Three red lights. When the wickets on a movable dam are down and there is an unobstructed navigable pass through the dam, the Inland Navigation Rules require that three red lights be displayed at point B on the illustration to mark the lock walls and piers. This alerts mariners that the lock is closed and the area is obstructed, even though the dam itself does not pose a hazard. The other answer choices are incorrect because one red light would not be sufficient to mark the obstruction, two red lights would not meet the regulatory requirement, and an amber light would not be the proper color to mark a closed lock.