Which of the following marine distress signals would be most effective when operating after dark or during restricted visibility?

The most effective marine distress signal when operating after dark or during restricted visibility is flares. Flares produce intense light and can be seen from a significant distance, making them an excellent choice for signaling distress. They are specifically designed to illuminate the area, drawing attention even in low visibility conditions. The bright flame of a flare can be visible for miles, especially in the dark, and their properties make them a standard in maritime distress signaling.

Other signals, such as sound signals, may be heard but do not provide visual confirmation of distress from a distance. Daytime flags are only effective during daylight and would not be visible after dark. Colored smoke signals, while useful during the day for visibility, would not be effective at night as their visibility would be dramatically reduced in the absence of light. Thus, flares stand out as the most reliable choice for nighttime or low visibility situations.