Which of the following is NOT a type of marine distress signal?

Raising a hand in greeting does not constitute a marine distress signal because it is commonly understood as a friendly gesture rather than a call for help. In maritime contexts, signals are specifically designed to convey distress or urgent messages to nearby vessels or rescuers.

Sounding a horn, casting a flare, and displaying a distress flag are all recognized means of signaling distress. The use of a horn in a continuous blast can attract attention and signal trouble, while flares provide a visible indication of distress that can be seen from a distance. A distress flag, often in a recognized format, is also a standard method to notify others that someone is in need of assistance. These methods are all aimed at ensuring quick communication of an emergency situation on the water, making their identification and use essential for safety at sea.