What is an indication that a boat may be in distress?

The correct answer is the indication of five or more short blasts on a horn, which is a recognized signal for distress on the water. This action conveys a message that the vessel is in trouble and may require assistance. The protocol is well-established in maritime communication, and other boaters and authorities are trained to respond to such signals promptly.

The other options do not carry the same universally understood meaning of distress. A flashing white light can indicate various situations, such as signaling night-time navigation or indicating location, but it does not specifically mean a vessel is in distress. Displaying a flag at half-mast typically signifies mourning or respect, not a distress call. The continuous ringing of a bell may be used for navigation signaling or as a warning but is not specifically designated as a distress signal, and thus may not alert others to the urgent need for help as effectively as the five short blasts do.