Explanation: Sea Area A1 is precisely defined by IMO GMDSS regulations as the area within the radiotelephone coverage of at least one VHF (Very High Frequency) coast station where continuous Digital Selective Calling (DSC) alerting is available. This typically extends 20-30 nautical miles from the coast, covering immediate coastal waters. Option B, Sea Area A2, refers to the area beyond A1, covered by MF (Medium Frequency) coast stations with continuous DSC alerting. MF offers a longer range than VHF. Option A, Ocean Area Regions (AOR-E, AOR-W, POR, IOR), are satellite coverage areas used by INMARSAT for Sea Areas A3 and A4, which cover vast ocean regions, not just VHF range. Option D, Coastal and Inland Waters, is a general description, but Sea Area A1 is the specific, internationally regulated GMDSS definition for the described VHF DSC coverage.