What is indicated if a vessel sees the sidelights of another vessel?

Answer: C — The vessels are crossing paths

When a vessel sees the sidelights of another vessel, it indicates that both vessels are crossing paths. According to the COLREGs, sidelights are seen when vessels are approaching each other at an angle, which suggests that they are not on parallel or reciprocal courses. This scenario typically requires the vessels to determine their respective responsibilities under the rules. In such situations, one vessel is categorized as the "stand-on" vessel, which should maintain its course and speed, while the other vessel, often considered the "give-way" vessel, must take action to avoid a collision. This is crucial for safe navigation, as the visibility of sidelights suggests an imminent encounter that must be managed according to the rules of the road.