Explanation: The frequency 500 kHz (0.5 MHz) was historically designated as the international radiotelegraph distress and calling frequency for stations in the mobile service, particularly maritime. It was primarily used for Morse code (A1A or A2B emissions) distress signals. Although modern Global Maritime Distress and Safety System (GMDSS) uses Digital Selective Calling (DSC) on other frequencies (like 2187.5 kHz and VHF channel 70) and satellite communications for primary distress alerting, 500 kHz remains the historically recognized and correct answer for questions specifically asking about the *radiotelegraph* distress frequency. The other options (300, 400, 600 kHz) are not designated international distress frequencies for this purpose.