Explanation: GMDSS imposes carriage requirements for different radio subsystems within a single sea area to ensure robust communication capabilities, particularly for distress and safety. The core principle is to mitigate the risk of a single point of failure. Different radio subsystems (e.g., VHF DSC, MF/HF DSC, satellite systems like Inmarsat, EPIRBs, SARTs) often operate on different frequencies, use varied propagation methods, and may have independent power sources. By combining these diverse systems, if one system or its primary power supply fails, other independent systems are still available to perform critical functions like sending a distress alert or receiving safety information. This "redundancy through diversity" significantly enhances the vessel's ability to communicate in an emergency, even if a component common to one system (like an antenna or a specific transceiver) becomes inoperable. A) While GMDSS incorporates redundancy, it doesn't necessarily duplicate *all* operational functions with *identical* systems. Instead, it uses different systems whose combined capabilities provide resilience. B) National regulations implement GMDSS, but the *reason* for these requirements originates from the international GMDSS framework's safety objectives, not merely to satisfy national rules. C) While effective communication with coast stations is vital, the primary reason for different *subsystems* is broader than just "all modes"; it's about providing multiple independent means of distress alerting and general safety communication.