What is a key safety measure regarding aircraft and their parked position?

The correct answer emphasizes the importance of securing aircraft while they are parked to prevent unintended movement. Placing chocks around the aircraft wheels is a critical safety measure designed to ensure that the aircraft remains stationary. This is especially vital on inclines or during adverse weather conditions, where the risk of rolling or shifting exists. By utilizing chocks, ramp agents help safeguard the aircraft and personnel working around it by ensuring that it cannot roll away.

In contrast, the other options focus on aspects that do not enhance safety in the same way. Having the aircraft unlocked can increase the risk of unauthorized access or accidents. Air circulators being turned off does relate to cabin safety and operational procedures but does not directly address the safety of the parked position. Similarly, maintaining a cabin that is unpressurized may be a standard operating procedure, but it does not pertain to the immediate risks associated with an aircraft's parked state. Thus, the requirement for chocks is the most direct and effective safety measure when managing parked aircraft.