Explanation: Aircraft Emergency Locator Transmitters (ELTs) are vital for search and rescue operations, and they operate on a combination of frequencies. Modern ELTs primarily transmit on **406 MHz**. This frequency sends a digitally encoded signal, including a unique identification and often precise GPS coordinates, to a satellite-based system (COSPAS-SARSAT). This enables rapid global detection and alerts rescue coordination centers. To aid local search and rescue teams once they are in the vicinity, 406 MHz ELTs also transmit a lower-power homing signal on **121.5 MHz**. Historically, 121.5 MHz was the primary international civilian distress frequency. **243 MHz** was the international military distress frequency, and many older ELTs would transmit on both 121.5 MHz and 243 MHz simultaneously. While 243 MHz is less common in new civilian ELTs, it remains a frequency utilized by some systems. Therefore, considering both historical devices and modern systems, ELTs can operate on 121.5, 243, and 406 MHz, making option D the most comprehensive and correct answer. Options A, B, and C are incomplete as they do not include all relevant frequencies used by ELTs for various stages of the search and rescue process.