Explanation: SELCAL, or SELective CALling, is an aeronautical communication system that significantly improves the efficiency of air-ground voice communications. Its primary function is to allow a ground-based radio operator to selectively alert a specific aircraft or group of aircraft that a call is waiting. When a ground station wants to contact an aircraft, it transmits a unique sequence of audio tones (a "four-tone burst") over the aircraft's assigned radio frequency. The aircraft's SELCAL decoder, which is programmed with its specific SELCAL code, continuously monitors the radio for this unique tone sequence. Upon detecting a match, it triggers an audible chime and/or a visual alert in the cockpit, prompting the flight crew to establish voice contact. This prevents the need for crews to constantly monitor noisy radio channels, especially over long flights or noisy HF frequencies, thereby reducing crew workload and fatigue. Option A is incorrect because SELCAL is an alerting system, not for receiver calibration or signal selectivity. Option C is incorrect; SELCAL is not an encryption system. The subsequent voice communication is typically unencrypted. Option D is incorrect because the system is initiated by a ground station to an aircraft and is not involved in calculating distance.