What is the duration of a prolonged blast of the whistle?

The duration of a prolonged blast of the whistle is indeed 4 to 6 seconds. This is an established standard in maritime navigation to convey specific intentions or warnings to other vessels. A prolonged blast is typically used to signal a vessel's presence, especially in restricted visibility situations, where communication becomes crucial for safety.

When a vessel sounds a prolonged blast, it serves as a clear and audible indication of its location and intentions to other vessels in the vicinity. This duration helps ensure that the signal is long enough to be recognized and understood by listeners and other vessels, thereby minimizing the risk of confusion that could lead to collisions or accidents at sea.

The other options reflect durations that do not meet the standardized requirement for a prolonged blast, leading to potential misinterpretation of the vessel's intentions.