When a power-driven vessel makes one prolonged blast, what is its purpose?

The purpose of a power-driven vessel making one prolonged blast is to signal a warning to other vessels. This sound is typically used in situations where the visibility is limited, such as in fog or heavy rain, or in crowded waterways where there may be confusion about the vessel's intentions or position. This prolonged blast helps to communicate the presence of the vessel to others nearby, ensuring safety and awareness on the water.

In contrast, the other choices relate to different signals and situations. Indicating a vessel is anchored is done through different sound signals, not a prolonged blast. Indicating when overtaking requires a specific sound sequence that is not applicable here. Lastly, reminding others of navigational rules is part of broader navigation practices but is not specifically communicated through a single prolonged blast.