Explanation: After an Inmarsat-C system is installed, entering your unique Inmarsat Mobile Number (IMN) into the terminal is a crucial first step. The IMN serves as the terminal's specific identifier within the Inmarsat satellite network, similar to an MMSI for a DSC radio. Without this number, the terminal cannot register with the network, establish communication, or be properly identified for routine messages or distress alerts as part of the Global Maritime Distress and Safety System (GMDSS). It's essential for the terminal's fundamental operation. * **A) Advise the nearest RCC that your terminal is active:** This is not required. RCCs manage distress incidents and vessel movements, not individual equipment installations. * **B) Send a distress alert and then cancel it:** This is a serious misuse of emergency systems and can lead to penalties. Distress alerts should never be sent for testing purposes. * **D) Send a brief test message to your own terminal:** While a good verification step, it cannot be done until the terminal is properly configured with its unique IMN and can register with the network.