The name given to an almost circular coral reef inside which there is a lagoon is called an atoll.

Option 1: Fringing reef refers to a reef that directly attaches to the shoreline or coast.

Option 2: Barrier reef, on the other hand, is a long coral reef parallel to the coast but separated from it by a deep channel or lagoon.

Option 3: Atoll is the correct answer. An atoll is a ring-shaped coral reef or island formed when a volcanic island sinks and leaves only the reef surrounding a lagoon.

Option 4: Isthmus is not related to coral reefs. It refers to a narrow strip of land that connects two larger land areas, usually with water on either side.

Therefore, the correct answer is option 3, atoll.