Which marine distress signal would be most effective when operating at night or during restricted visibility?

When operating at night or during restricted visibility, flares serve as one of the most effective marine distress signals due to their bright, intense light that is easily visible over long distances. Flares can cut through darkness and can be seen from miles away, making them particularly useful in emergencies when visibility may be compromised, such as during fog or nightfall.

In contrast, while flags, sound signals, and reflective mirrors each have their own purposes and situations where they are effective, they do not provide the same level of visibility at night. Flags rely on visual recognition in daylight, sound signals depend on audible reception, and reflective mirrors require sunlight to function properly. Therefore, the use of flares is considered the most reliable and effective method for signaling distress in conditions of low visibility.