What is defined as the interface between land and sea?

The correct answer is the coast, which refers to the area where land meets the sea. This zone is characterized by various geographical features, including cliffs, beaches, dunes, and estuaries, depending on the local environment and geological processes. Coasts play a vital role in ecosystem dynamics, offering habitats for various marine and terrestrial species. Additionally, they are significant for human activities such as tourism, fishing, and shipping.

An estuary, while related to the interaction of land and water, specifically refers to the tidal mouth of a river, where fresh and saltwater mix. A beach is a specific type of coastal landform made up of loose particles like sand or gravel, but it does not constitute the entire interface between land and sea. A harbour is a man-made or naturally occurring body of water where ships dock or anchor, which exists within the coastal zone but does not define the broader interface itself.