Which of the following carbonates is the most stable?

Thermal stability order:

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

As the size of cation increases, thermal stability of carbonate increases
Since a large size cation [Ba⁺²] will stabilises the large size anion [CO₃^-2]