⇒ The thermal stability increases with increase in the electropositive character of the metal or the size of the cation present in the carbonate increases.

⇒ The increasing order of the size of cations of the given carbonates:

Mg < Ca < Sr < Ba

Hence, the increasing order of the thermal stability is:

MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃

⇒ Thus, among alkaline earth metal carbonates, option (D), BaCO₃ is thermally the most stable.