Explanation: GMDSS regulations mandate specific maintenance requirements to ensure the continuous operational readiness of critical safety equipment. Option A correctly identifies the three recognized methods for fulfilling GMDSS maintenance requirements: 1. **Duplication of equipment:** Having redundant critical GMDSS equipment (e.g., two complete Inmarsat C systems) ensures a backup is immediately available if the primary fails. 2. **At-sea maintenance:** This involves carrying qualified GMDSS maintainers onboard, or having specialized test equipment and spares to allow the vessel's crew to perform repairs while at sea. 3. **Shore-based maintenance:** Relying on qualified shore-based technicians to service the equipment when the vessel is in port. All compulsory GMDSS vessels must provide *at least two* of these three options. Option B is incorrect because all compulsory GMDSS vessels, regardless of tonnage, must provide at least two maintenance options, not just one. Option C is incorrect. The "at-sea" maintenance option is about the capability to repair equipment onboard, which often includes carrying a qualified maintainer or appropriate test gear and spares. The number of licensed GMDSS operators onboard does not waive this requirement; rather, having qualified operators is a separate operational requirement. Option D is incorrect. While compulsory GMDSS vessels must carry qualified operators, the specific requirement is generally for *at least two* qualified persons (e.g., GMDSS Radio Operators or Restricted GMDSS Radio Operators), not just one, and this is an operational manning requirement, not a maintenance requirement tied exclusively to Sea Area A4.