Explanation: Digital Selective Calling (DSC) functions, particularly for emergency signaling, rely on a unique identifier for your vessel. The 9-digit Mobile Maritime Service Identity (MMSI) serves as this unique electronic "address" for your vessel within the global maritime communications system. When a DSC distress alert is sent, it *must* include the vessel's MMSI so that rescue authorities and other vessels can identify the source of the distress and respond appropriately. Without a programmed MMSI, the radio cannot generate a valid DSC message, thus preventing the activation of its emergency signaling capabilities. While GPS position input (C) is extremely important for a *useful* DSC distress alert, allowing rescuers to locate you, the radio's fundamental ability to transmit a DSC message and identify itself first requires the MMSI. Options A and B are incorrect; a marine VHF requires a single 12-volt power source and generally uses one antenna for both voice and DSC.