The correct order of increasing thermal stability of K2CO3, MgCO3, CaCO3 and BeCO3 is:
BeCO3 < MgCO3 < CaCO3 < K2CO3
Within the same group, the thermal stability of the carbonates increases as we go down the group.
If you compare the carbonates of alkali and alkaline earth metals of the same period, the alkali earth metal carbonate is more stable. Watch the video for more.