Explanation: The term "Reserve Source of Energy" specifically describes the dedicated power supply, typically batteries, required to energize the GMDSS console and associated equipment should both the ship's main electrical power and its emergency generator fail. GMDSS regulations (e.g., IMO SOLAS and FCC Part 80) mandate this independent power source to ensure essential communication capabilities remain available for a specified duration (e.g., 1 or 6 hours) during extreme emergencies. A) "Emergency power" is a broader term that encompasses the ship's general emergency generator, which is intended as the *first* backup to main power. The question specifies failure of *both* main and emergency energy. B) The "Ship's emergency diesel generator" is a common source of the ship's general emergency power. The question implies this has also failed. C) "Ship's standby generator" is often synonymous with an emergency generator or refers to non-essential backup. It doesn't denote the specific, regulatory-mandated GMDSS power source after all other ship's power has failed.