Carbonates of all alkaline earth metals decompose to give metal oxide and carbon dioxide. The thermal stability of a metal carbonate depends on the cationic size, it increases with increasing cationic size. The carbonates form the positively charged metal cation and negatively charged CO₃^2-. The larger the cationic size, the more electropositive is the ion. The electropositivity of the alkaline earth metals increases down the group from Be to Ba. Due to these reason, BaCO₃ is the most stable metal carbonate.

The correct option is (iv).