Which of the following is considered a distress signal?

A dye marker is widely recognized as a distress signal in maritime situations. When deployed, it may create a highly visible color in the water, making it easier for rescuers to locate a person or vessel in distress. Its effectiveness is enhanced during search and rescue operations, as the bright color can be seen from a distance and can attract attention from aircraft and nearby vessels.

While other options may have specific uses in signaling or communication, they do not carry the same international recognition as a distress signal as the dye marker does. A flag with a cross may signify different messages depending on the context, a single whistle blast is typically not sufficient alone to indicate distress, and a bright flashlight is useful for signaling but is not classified as an established distress signal on its own. Thus, the dye marker remains the most definitive option among the provided choices for signaling distress.