The correct answer is D) "P". The signal flag "P" (Pennant) is used to inform a vessel's crew to report on board as the vessel is about to proceed to sea. This is a standard signal used in maritime operations to indicate that the vessel is preparing to depart and all crew members should be aboard and ready. The other options are incorrect because: A) "A" is the "Diver Down" signal, B) "H" is the "Pilot" signal, and C) "U" is the "You should stop your vessel immediately" signal. None of these alternatives are the appropriate signal to indicate that a vessel is about to depart from the port.