Which of the following terms signifies an emergency that requires immediate assistance?

The term that signifies an emergency requiring immediate assistance is "MAYDAY." This term is internationally recognized in aviation and maritime communication as the universal distress signal. When a pilot or mariner issues a "MAYDAY" call, it indicates a life-threatening emergency that demands urgent attention and immediate response from other vessels or aircraft.

In contrast, "PAN PAN" is used to indicate a situation that is urgent but not immediately life-threatening. It signifies a need for assistance, but the severity is less critical than a "MAYDAY" call. "STANDBY" is typically a request for the receiving station to hold off on communications or to wait, indicating no immediate danger. The term "EMERGENCY" is more general and may describe a situation requiring assistance, but it does not specify the level of urgency associated with such a call like "MAYDAY" does. Hence, "MAYDAY" is the most appropriate term for an emergency situation requiring immediate help.