Answer: The correct option is b. Calcium
Explanation: Ocean water consists of a large amount of Chloride \((Cl^-)\), Sulfate \((SO_4^{2-})\), Magnesium \((Mg^{2+})\), Sodium \((Na^+)\), Calcium \((Ca^{2+})\) and Potassium \((K^+)\) ions.
The coral animals absorb calcium element from the ocean to form their skeleton. Then, the calcium ions combine with carbon and oxygen to form calcite.
Hence, the correct option is b.
