Final answer:
The flashing white light gun signal is used to communicate with aircraft when radio communication is not possible, indicating an unsafe airport for landing to airborne aircraft or instructing grounded aircraft to return to the starting point.
Step-by-step explanation:
The flashing white light gun signal in aviation is used primarily for communication with aircraft and vehicles when radio contact is not possible. Specifically, a flashing white light directed at an aircraft in flight means that the airport is unsafe for landing, and the pilot should not attempt to land. However, when directed at an aircraft on the ground, a flashing white light indicates that the aircraft should return to the starting point on the airport.
The correct answer to the question is therefore not listed as an option. The purposes provided in the options do not accurately match the standard aviation light signals.