Explanation: Marine Digital Selective Calling (DSC) VHF radios require a constant stream of GPS data (often in NMEA 0183 or NMEA 2000 sentences) to function correctly, especially for distress calls and position reporting. This data includes your vessel's precise latitude and longitude, and often speed over ground. The most direct and reliable way to verify that the GPS receiver is correctly connected and transmitting valid data to the VHF radio is to observe the radio's display. If the GPS data stream (the "sentence") is being received and interpreted properly, the radio will typically show your current latitude, longitude, and sometimes speed. This visual confirmation ensures the interface is working as intended. Pressing the distress button (B) is an emergency action and should never be used for testing. A generic "GPS confirmation readout" (C) might indicate a signal is present but doesn't verify the specific, critical navigation data (Lat/Lon) is being correctly processed. Asking for a radio check position report (D) is a verbal communication and doesn't confirm the electronic data flow from the GPS to the radio's internal systems.