Explanation: When the "At-Sea maintenance method" is chosen for GMDSS equipment aboard a compulsory vessel, the International Convention for the Safety of Life at Sea (SOLAS) requires that the ship carry at least one person qualified to maintain the radio equipment. This requirement is consistent across all GMDSS sea areas (A1, A2, A3, and A4). The purpose of this maintainer is to ensure the continuous availability of GMDSS facilities while the ship is at sea by performing necessary repairs. Therefore, carrying one GMDSS radio maintainer satisfies the requirement regardless of the specific sea area of operation, making options B and C incorrect as they specify two maintainers or limit the requirement to certain sea areas.