The correct answer is C) Slow your vessel and keep well clear because she has a diver down. The international signal flag "A" (also known as the "Alpha" flag) indicates that a vessel has a diver down or that diving operations are in progress. When you see this flag displayed, you should slow your vessel and keep a safe distance to avoid interfering with the diving operation and endangering the diver. The other options are incorrect because: A) The "A" flag does not indicate the vessel is carrying dangerous goods, B) The "A" flag does not indicate the vessel is disabled, and D) You should not stop your vessel instantly, as that could create an unsafe situation.