Explanation: 518 kHz is designated as the primary international NAVTEX frequency. This medium frequency (MF) is exclusively used for broadcasting critical navigational and meteorological warnings, search and rescue information, and other urgent safety messages to ships worldwide. It forms an integral part of the Global Maritime Distress and Safety System (GMDSS) for disseminating essential maritime safety information. Conversely, 2187.5 kHz and 4209.5 kHz are Digital Selective Calling (DSC) frequencies used for automated distress, urgency, and safety *calling* on the MF and HF bands, respectively, not for receiving NAVTEX broadcasts. VHF channel 16 is the international voice distress and calling channel for short-range communication. These alternatives serve different functions within maritime communication regulations.