Explanation: GMDSS (Global Maritime Distress and Safety System) regulations, specifically SOLAS Chapter IV, mandate a tiered approach to power supply for critical radio communications. 1. **Main Source:** The primary electrical power for the ship. 2. **Emergency Source:** If the main source fails, an emergency source (e.g., an emergency diesel generator or a large battery bank) must automatically take over to power essential services, including GMDSS equipment. 3. **Reserve Source of Energy:** The question specifies the failure of *both* the main and emergency power sources. In this critical scenario, GMDSS equipment must be supplied by an independent "reserve source of energy." This is typically a dedicated bank of batteries, separate from the ship's main and emergency power systems, designed to power the GMDSS console and essential radio equipment for a specified minimum duration (e.g., 1 or 6 hours, depending on the ship's design and emergency power capabilities). Its sole purpose is to ensure continued distress and safety communications in extreme emergencies. Options A, B, and D refer to the "emergency power" system or components thereof, which the question explicitly states has already failed. The "reserve source of energy" is the *final* independent power supply specifically for GMDSS operations in such a catastrophic event.