BOTH INTERNATIONAL & INLAND What does five or more short blasts on a vessel's whistle indicate?

Correct Answer: Option D — There is doubt that another vessel is taking sufficient action

Rule 34(d) defines the signal of five or more short and rapid blasts as the 'doubt signal.' It is used when a vessel is in doubt as to whether sufficient action is being taken by another vessel to avoid collision or when the other vessel's intentions are not understood.