What lights may a sailing vessel underway exhibit?

A sailing vessel underway must display specific lights to ensure safe navigation and communicate its status to other vessels. The correct answer involves a red light over a green all-round light. This configuration indicates that the vessel is under sail and is showing its capacity to maneuver as prescribed by the COLREGs.

The red light represents the port side, while the green light signifies the starboard side of the vessel. The combination of these two colored lights allows other vessels to identify the sailing vessel's direction and provide important information about its navigation status. By presenting these lights, the sailing vessel adheres to the regulations that enhance visibility and promote safety at sea during nighttime operations or reduced visibility conditions.

The other options described do not meet the requirements for a sailing vessel underway. An all-round white light alone is insufficient, as it doesn’t convey information about the vessel's course or maneuverability. Two vertical all-round lights are indicative of a vessel engaged in fishing or other specific operations, not of a sailing vessel. A green light over a white light does not correspond to any standard configuration recognized in the COLREGs for sailing vessels.