Explanation: 518 kHz is indeed the designated international NAVTEX frequency for broadcasting navigational warnings, meteorological forecasts, and other urgent safety information to ships in English. This frequency is a critical component of the Global Maritime Distress and Safety System (GMDSS), ensuring mariners receive essential updates worldwide. 2187.5 kHz is the international Digital Selective Calling (DSC) distress and safety frequency on the medium frequency (MF) band, used for initiating distress alerts, not for NAVTEX broadcasts. 4209.5 kHz is another international NAVTEX frequency, often referred to as a secondary or supplementary frequency, used for additional services or national language broadcasts, but 518 kHz remains the primary international frequency for English messages. VHF channel 16 (156.8 MHz) is the international voice distress, urgency, and safety calling frequency on the VHF band, while 2187.5 kHz, as mentioned, is for MF DSC distress. Neither is used for NAVTEX broadcasts, which is a specialized receive-only data service.