The correct order of increasing thermal stability of K2CO3, MgCO3, CaCO3 and BeCO3 is:

Since a large size cation(K+) will stabilise the large size anion [CO3-2]Hint :   If both factors vary at same time then charge is dominant over radius Size order : K+ > Ca+2Charges order Ca+2 > K+In the above example both size and charge varies but charge dominant over radius Stability order : K2CO3 > CaCO3