What is defined as the operational evolution where an aircraft is refueled while the engine is operating?

Hot refueling refers to the process of refueling an aircraft while its engines are still running. This operational evolution is particularly essential in military aviation, where minimizing downtime and ensuring rapid turnaround is critical for mission success. Conducting hot refueling allows for quicker operations, enabling aircraft to remain in action without unnecessary interruptions.

In contrast, cold refueling occurs when the aircraft's engines are shut down, which typically involves a more prolonged process due to the need for engine rest and the assurance of safety protocols. Safety refueling is not a standard term associated with this specific operational process, and "NORMAL" refueling may not convey any particular operational context as compared to hot refueling. Therefore, hot refueling is the correct choice as it directly pertains to the scenario of refueling with the engines operating, highlighting the necessity of efficiency in combat and fast-paced environments.