Which of the following is a visual distress signal for use at night?

The use of an electric light as a visual distress signal at night is correct because it can effectively draw attention in low-light conditions. Electric lights can include devices such as strobes or flashlights, which are visible from a distance and can alert nearby vessels or rescuers to a boat in distress. They are commonly accepted as a means to signal for help at night when visibility is limited.

In contrast, options like a green flare are generally designated for daytime use rather than night. Sound signals, while useful for communication, do not provide a visual indication and therefore are not effective as a nighttime distress signal. Similarly, a daytime flag is specifically intended for use during daylight hours when visibility is better, making it unsuitable for nighttime communication. Thus, the electric light stands out as the best choice for an effective nighttime visual distress signal.