Explanation: The Distance Measuring Equipment (DME) system calculates distance by measuring the time difference between an aircraft's interrogation pulse and the ground station's reply. At very close ranges, the actual propagation time (the time for the radio signal to travel to and from the aircraft) would be extremely short, potentially near zero. A built-in, fixed delay (typically around 50 microseconds) in the ground station's reply ensures that there is always a sufficiently long time interval for the aircraft's DME receiver to accurately measure. Without this delay, at close ranges, the measured time might be too short to resolve accurately, or it could be confused with the aircraft's own transmitted pulse. The aircraft's DME unit is designed to subtract this known, constant delay from the total measured time to determine the true round-trip propagation time, allowing for accurate distance calculation even when operating very close to the ground station. Options A, B, and D describe irrelevant or incorrect functions regarding DME system timing and operation.