Which of the following is NOT a valid distress signal?

Waving a white flag is not considered a valid distress signal according to the International Maritime Organization. The white flag is predominantly recognized as a signal of truce or surrender rather than indicating distress or a need for assistance.

In contrast, firing a red flare is a well-established distress signal. Red flares are internationally recognized as indicators of emergency and are used to attract attention in situations where help is needed. Similarly, displaying a square flag with a ball above it is identified as the signal for distress under the conventions of mariners, more commonly recognized as the "Code Flag NP." Lastly, using SOS in Morse code—three dots, three dashes, three dots—is universally acknowledged as a distress signal and can be transmitted in various forms, including light or sound.

Each of these signals plays a crucial role in maritime safety by clearly communicating a vessel's need for assistance, whereas waving a white flag lacks that specific connotation in the context of distress.