Atoll is a type of landform that is commonly found in tropical and subtropical areas. It is formed by the growth of coral reefs on top of submerged volcanic mountains. Over time, as the volcano subsides, the coral reefs continue to grow and form a circular or oval-shaped barrier reef. The center of this reef becomes a lagoon, surrounded by a ring of coral islands or islets. This entire landform is called an atoll.

Option 1: A sand-dune refers to a hill of sand formed by wind or water. This is not the correct definition for an atoll.

Option 2: A lake is a large body of water surrounded by land. This does not accurately describe an atoll, which is primarily composed of coral reefs.

Option 3: A sandbar is a landform made of sand that is deposited by waves or currents. While atolls may have sandy areas, they are not exclusively composed of sandbars.

Option 4: The correct answer. An inland sea refers to a large body of saltwater that is surrounded by land. This is not an accurate description of an atoll as it is primarily formed by coral reefs and not by the presence of an inland sea.