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The correct order of increasing thermal stability of $K_2CO_3$, $MgCO_3$, $CaCO_3$ and $BeCO_3$ is
A
$BeCO_3 \lt MgCO_3 \lt CaCO_3 \lt K_2CO_3$
B
$MgCO_3 \lt BeCO_3 \lt CaCO_3 \lt K_2CO_3$
C
$K_2CO_3 \lt MgCO_3 \lt CaCO_3 \lt BeCO_3$
D
$BeCO_3 \lt MgCO_3 \lt K_2CO_3 \lt CaCO_3$
Detailed Solution
A small, highly charged cation polarises the large carbonate ion strongly and helps it break into the oxide and $CO_2$.
As the basicity of the metal hydroxides increases down the group from Be to Ba (the cation becomes larger and less polarising), the thermal stability of their carbonates also increases in the same order.
So among the group 2 carbonates: $BeCO_3 \lt MgCO_3 \lt CaCO_3$
Further, group 1 compounds are more thermally stable than group 2 compounds, because their hydroxides are much more basic than group 2 hydroxides; the large unipositive $K^+$ ion has very little polarising power.
$K_2CO_3$ does not decompose even on strong heating, so it is the most stable.
Therefore the order of thermal stability is $BeCO_3 \lt MgCO_3 \lt CaCO_3 \lt K_2CO_3$.
As the basicity of the metal hydroxides increases down the group from Be to Ba (the cation becomes larger and less polarising), the thermal stability of their carbonates also increases in the same order.
So among the group 2 carbonates: $BeCO_3 \lt MgCO_3 \lt CaCO_3$
Further, group 1 compounds are more thermally stable than group 2 compounds, because their hydroxides are much more basic than group 2 hydroxides; the large unipositive $K^+$ ion has very little polarising power.
$K_2CO_3$ does not decompose even on strong heating, so it is the most stable.
Therefore the order of thermal stability is $BeCO_3 \lt MgCO_3 \lt CaCO_3 \lt K_2CO_3$.