Final answer:
The Code Alfa flag indicates 'diver down,' which means that there is a diver currently submerged in the area, and it alerts nearby vessels to navigate with caution.
Step-by-step explanation:
The Code Alfa (or Alpha) flag signifies a 'diver down,' which indicates that a diver is currently submerged in the vicinity. This is distinct from the options provided in the question (Man overboard, Fire on board, Abandon ship, Medical emergency). The Alfa flag is internationally recognized, and vessels nearby are expected to keep clear and navigate at a slow speed to ensure the safety of the divers in the water.