Thermal stability refers to the ability of a substance to resist decomposition due to heat. The carbonate of alkali metals decompose on heating to give the corresponding oxide and carbon dioxide gas. The thermal stability of these carbonates increases down the group as the size of the cation increases.

Explanation:
- Lithium carbonate (Li2CO3) has the least thermal stability among the given options.
- This is because lithium is the smallest cation among the alkali metals and therefore has the highest charge density.
- Due to this high charge density, the carbonate ion (CO3 2-) is strongly attracted towards the lithium ions, making it difficult to break the bond between them.
- As a result, lithium carbonate requires less energy to decompose and has the least thermal stability among the given options.
- On the other hand, as we move down the group, the size of the cation increases, leading to a decrease in the charge density and a decrease in the strength of the attraction between the carbonate ion and the cation.
- Therefore, the thermal stability of the carbonates increases down the group.

Summary:
- Lithium carbonate (Li2CO3) has the least thermal stability among the given options.
- This is because of the high charge density of lithium ion, which makes it difficult to break the bond between the carbonate ion and the lithium ion.
- Thermal stability of the carbonates increases down the group due to the decrease in the charge density of the cation.