Thermal stability of metal carbonates increases down the group in the periodic table. This is because larger cations (down the group) have lower polarizing power and hence stabilize the carbonate ion better, making them more thermally stable.
- Group 2 elements: Mg, Ca, Sr, Ba.
- Order of size: Mg < Ca < Sr < Ba.
- Therefore, their carbonates' thermal stability increases in this order.
Hence, the correct order of increasing thermal stability is:
\[
\text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3
\]