The correct order of thermal stability is:

The correct order of thermal stability is ${\mathrm{BeCO}}_3<{\mathrm{MgCO}}_3<{\mathrm{CaCO}}_3<{\mathrm{SrCO}}_3<{\mathrm{BaCO}}_3$. In all circumstances, given a certain group of, Compounds from Group 1 or Group 2 become more thermally stable as the cation's ionic radius grows and its polarizing power falls.

Because the cation has a greater ionic radius and a lower polarizing power due to its bigger ionic radius and lesser charge, group 1 compounds are often more thermally stable than group 2 compounds. comparing neighboring metals from the same time is especially important.

Consequently, thermal stability increases in the following order:

${\mathrm{BeCO}}_3<{\mathrm{MgCO}}_3<{\mathrm{CaCO}}_3<{\mathrm{SrCO}}_3<{\mathrm{BaCO}}_3$.

Therefore, option A is correct.