In the case of a man overboard, what is the appropriate signal?

The appropriate signal in the case of a man overboard is "Pan Pan Pan." This indication is used to alert other vessels that there is an urgent situation that requires assistance, but it is not as critical as a life-threatening emergency. A person overboard is a serious situation, and signaling with "Pan Pan" communicates that immediate help is needed to rescue someone, without the conditions being life-threatening at that very moment.

The other choices serve different purposes: "Mayday Mayday Mayday" is reserved for grave and imminent danger where life is threatened, and using this signal indicates a dire situation. "Distress Distress Distress" is a similar global phrase to "Mayday" that denotes a life-threatening emergency. "Send Help Immediately" does not conform to the standardized communication protocols used internationally for marine communications and lacks the recognized urgency of the "Pan Pan" signal.