A chain or group of islands scattered in a sea is called an:

An archipelago is a group or chain of islands clustered together in a sea or ocean. The word comes from the Greek "arkhi" (chief) and "pelagos" (sea); it originally referred to the Aegean Sea, which is dotted with many islands.

How archipelagos form:

Volcanic activity: Chains of volcanic islands form over hotspots or along plate boundaries. Examples: Hawaii (hotspot), Japan and the Aleutian Islands (plate boundaries).

Coral growth: Coral reefs and atolls form island groups in warm tropical seas. Examples: the Maldives and Lakshadweep.

Continental fragments and rising sea levels: Parts of the mainland become separated as sea levels rise, such as the islands off the coast of Norway and Scotland.

Erosion and plate movements can also create island groups.

Largest archipelago countries:

• Indonesia: with more than 17,000 islands, it is the largest archipelagic country in the world.
• The Philippines: with more than 7,600 islands.
• Japan: with about 14,000 islands.

Other famous archipelagos:

• The Galapagos Islands (Ecuador), famous for Charles Darwin's studies.
• The Canary Islands (Spain).
• The Azores (Portugal).
• The Canadian Arctic Archipelago, the largest archipelago by land area.
• The Malay Archipelago, the largest archipelago in the world, including Indonesia and the Philippines.

Archipelagos of India:

• The Andaman and Nicobar Islands in the Bay of Bengal, with about 836 islands, islets and rocks.
• The Lakshadweep Islands in the Arabian Sea.
• Ritchie's Archipelago, a group of islands east of Great Andaman, which includes Havelock Island (Swaraj Dweep).

Archipelagic states:

Under the UN Convention on the Law of the Sea (UNCLOS), countries made entirely of islands, such as Indonesia and the Philippines, can draw "archipelagic baselines" around their outermost islands and treat the waters inside as their own.

Why not the other options?

• ✗An atoll is a ring-shaped coral reef or island enclosing a lagoon.
• ✗A lagoon is a shallow body of water separated from the sea by a reef or sandbar.
• ✗An isthmus is a narrow strip of land connecting two larger landmasses.