The correct answer is C) Five or more short rapid blasts on the whistle. This is the correct answer because the International Regulations for Preventing Collisions at Sea (COLREGS) and the Inland Navigation Rules both state that the danger signal is five or more short, rapid blasts on the vessel's whistle or horn. This signal is used to indicate that a vessel is in distress and requires immediate assistance. The other options are incorrect because: A) A continuous sounding of the fog signal is used to indicate the presence of a vessel in restricted visibility, not a danger signal. B) Firing a gun every minute is not a standard navigational signal. D) One prolonged blast on the whistle is used to indicate a vessel's change of direction or position, not a danger signal.