The correct answer is A) Attempt to call the vessel on VHF radiotelephone and begin a search because the vessel has a man overboard. The signal flag "O" (Oscar) is the international signal for "Man Overboard". When a vessel displays this flag, it indicates that a person has fallen overboard and immediate assistance is required. The proper action is to attempt to contact the vessel on VHF radio, slow down, post lookouts, and begin searching the area for the person in the water. The other answer choices are incorrect because they do not properly address the emergency situation indicated by the "O" flag. Approaching with caution, proceeding on course, or assuming the vessel is disabled would not be the appropriate response to a man overboard incident.