The correct answer is B) a vessel trawling. The International and Inland Navigation Rules (commonly known as the "Rules of the Road") require a vessel engaged in trawling, which is a commercial fishing method that involves pulling a fishing net through the water, to display a green light over a white light in a vertical line above the level of the sidelights. This arrangement of lights helps other vessels identify the vessel as engaged in trawling, which may be restricted in its ability to maneuver. The other options are incorrect because a pilot vessel is required to display different lights, a sailboat under sail and power is not required to display this specific light configuration, and a vessel engaged in underwater construction is not specifically addressed by this rule.