When approaching another vessel at night and seeing red and green sidelights along with three white lights in a vertical line, what vessel is this likely to be?

Answer: B — vessel towing a tow more than 200 meters astern

The configuration of red and green sidelights combined with three white lights arranged in a vertical line indicates that you are likely encountering a towing vessel engaged in towing operations, particularly a vessel towing a tow that is more than 200 meters in length. According to the International Regulations for Preventing Collisions at Sea (COLREGS), a towing vessel displays these lights to indicate its status and the nature of its operations, especially at night when visibility is limited. The red and green sidelights are standard navigational lights that indicate the port (left) and starboard (right) sides of a vessel. The three white lights in a vertical arrangement, typically displayed by a towing vessel, signify that it is engaged in towing. The two white lights above the side lights indicate that it is also towing a long tow, fulfilling the requirement to signal its particular situation. This is essential for other vessels to understand the towing arrangement, especially if the tow exceeds 200 meters, as it may require extra caution and maneuvering space. The other options do not present the same combination of lights; for example, a sailing vessel under sail would only display sidelights and a sternlight without the vertical white lights, while fishing vessels have different light configurations, usually involving green and white