Question

atolls are geological formations made up of:

animal skeletons.
mountains.
coral.
island chains.

Explanation:

Define geological composition of atolls

An atoll is a ring-shaped coral reef, island, or series of islets that surrounds a body of water called a lagoon. They are built by coral polyps in warm tropical waters.

Evaluate the given options

• animal skeletons: Too broad; while corals are marine invertebrates, atolls are specifically characterized by coral structures.
• mountains: Atolls are low-lying structures built on submerged volcanic foundations, not made of mountains.
• coral: Correct, as atolls are primarily formed from organic coral rim accumulations.
• island chains: Atolls can form chains, but they themselves are made of coral.

Answer:

• animal skeletons.
• mountains.
• coral. (Correct answer)
• island chains.