On heating, which of the following releases CO2 most easily?

On heating, MgCO3 releases CO2 most easily. This is due to the small size of the resulting oxide ion. The radii of the oxide ion and Mg2+ ion are nearly the same, and hence they pack together tightly. As a result, its formation (i.e., decomposition of its carbonate) is easy and requires less energy. Hence the correct option is MgCO3.