Which signaling device can be used in an emergency at sea?

In an emergency situation at sea, a signaling device is crucial for attracting attention and communicating distress. A whistle or mirror serves this purpose effectively. A whistle can produce a loud sound that carries over distances and can be easily used to alert nearby vessels or rescuers to your presence. Similarly, a mirror can reflect sunlight to create a visible signal that can be seen from afar, especially in bright conditions. Both of these items are simple, portable, and specifically designed for signaling, making them invaluable tools during an emergency.

The other options do not serve the primary purpose of signaling. A primary navigation light is designed for illumination and navigation rather than communication. A handheld compass is useful for navigation but does not assist in signaling for help. A fishing rod is primarily meant for catching fish and has no function in signaling an emergency situation at sea.