Explanation: The primary international frequency used exclusively for NAVTEX (Navigational Telex) broadcasts is 518 kHz. This frequency is in the Medium Frequency (MF) band and is globally recognized for transmitting maritime safety information (MSI), including navigational warnings, meteorological forecasts, and search and rescue information, in English. * **A) 2187.5 kHz:** This is the international digital selective calling (DSC) frequency for distress, urgency, and safety in the High Frequency (HF) band, not for NAVTEX data broadcasts. * **B) 4209.5 kHz:** While an HF NAVTEX frequency (part of the global system for long-range broadcasts), it is not the *primary* international NAVTEX frequency. The primary frequency is in the MF band. * **C) VHF channel 16:** This is the international voice calling and distress frequency on Very High Frequency (VHF) for short-range communications, not a data broadcast frequency for NAVTEX.