Which of the following is a recognized distress signal?

The recognized distress signal among the options listed is a red flare. A red flare is an internationally recognized, visual distress signal used in maritime situations to indicate that a vessel or someone aboard is in need of assistance. When fired, it sends up a bright, red flame that can be seen over great distances, attracting the attention of nearby vessels or aircraft.

While three whistle blasts and continuous sounding of a fog horn are capable of conveying messages, they are not formal distress signals recognized by international maritime convention. One short horn blast typically indicates a maneuvering action, such as turning to starboard, rather than a call for help. Therefore, among the given choices, the red flare is distinctly acknowledged as an effective and standard means of signaling distress at sea.