Explanation: Reserve power batteries on vessels subject to Global Maritime Distress and Safety System (GMDSS) requirements are critical for maintaining essential radio communications during a main power failure. To ensure their reliability, regulations mandate periodic testing. Answer D is correct because FCC regulations, specifically 47 CFR Part 80, Subpart W, state that the reserve source of energy must be tested at intervals not exceeding 12 months, or during the vessel's inspection. This test typically involves a discharge with the normal operating load connected for a specified duration (e.g., one hour) to verify the battery's capacity and operational readiness under emergency conditions. Answers A, B, and C suggest incorrect regulatory intervals for this comprehensive discharge test. While more frequent, less intensive checks (like voltage monitoring) might be part of good operating practice, the full regulatory test to confirm reserve power capability is mandated annually or during the official inspection.