The electropositive feature is stronger the further down the alkali metal group we go. Alkali carbonates become more stable as a result of this. Lithium carbonate, however, is not very heat stable.

This is due to the covalent nature of lithium carbonate. Due to its tiny size, lithium ions may polarize larger carbonate ions, creating the more stable lithium oxide.

Only ${\mathrm{Li}}_2{\mathrm{CO}}_3$ of the alkali metal carbonates breaks down into CO₂ and metal oxide.

${\mathrm{Li}}_2{\mathrm{CO}}_3\to {\mathrm{Li}}_2\mathrm{O}+{\mathrm{CO}}_2\uparrow$

Hence the option D is correct.