Which of the following does NOT represent a valid distress signal?

The identification of a long scream as a distress signal is not standard in emergency protocols. Recognized distress signals are typically brief, easily repeatable actions that can be recognized from a distance. For example, discharging a firearm three times in quick succession, creating three fires that are evenly spaced, or producing three short blasts from a whistle are all widely understood signals indicating that someone is in trouble and requires assistance. These methods provide clear and recognizable patterns that can be noticed by rescuers.

In contrast, a single long scream lacks the defined structure and repeatability that characterizes effective distress signals. It can also be less audible over distances or in environments where other sounds might drown it out, making it an unreliable option for signaling distress. Therefore, while the other choices provide established means of communication to signal for help, a long scream does not fulfill the criteria for a valid distress signal.