Explanation: The Global Maritime Distress and Safety System (GMDSS) divides the world's oceans into "Sea Areas" based on the available communication capabilities for distress and safety. * **Sea Area A1** is within range of a VHF-DSC coast station. * **Sea Area A2** is beyond A1 but within range of an MF-DSC coast station. * **Sea Area A3** is beyond A2 but within range of an Inmarsat geostationary satellite. * **Sea Area A4** covers the remaining polar regions, primarily served by HF-DSC. Therefore, the specific GMDSS communication infrastructure available at a given geographic location—i.e., whether it's covered by VHF-DSC, MF-DSC, HF-DSC coast stations, or Inmarsat satellites—determines which Sea Area it is. Options A (distance from shore) is a factor in some areas but not the definitive, universal determinant. Options B (maintenance availability) and D (voyage type) are unrelated to the regulatory definition of Sea Areas.