Explanation: The COSPAS-SARSAT system is an international satellite-based search and rescue alert system. Its main function is to detect and locate distress beacons, specifically EPIRBs (Emergency Position-Indicating Radio Beacons), ELTs (Emergency Locator Transmitters), and PLBs (Personal Locator Beacons). These modern beacons transmit a digital signal on 406 MHz, which is specifically designed for satellite reception. The 406 MHz signal provides unique identification data and often includes GPS coordinates, allowing for rapid and accurate location of a distress incident. Option A is incorrect because COSPAS-SARSAT no longer processes 121.5 MHz signals directly from satellites; this frequency is now mainly used for local homing by rescue forces. Options C and D refer to frequencies (1635 MHz and 2187.5 kHz) that are not used by the COSPAS-SARSAT system for satellite detection of distress beacons. 2187.5 kHz is a maritime DSC (Digital Selective Calling) frequency, part of a terrestrial communication system, not satellite detection.