Which of the following carbonates decomposes readily at low temperatures?
The correct option is B $$Li_2CO_3$$
As we move down the group, the electropositive character increases. This causes an increase in the stability of alkali carbocation.
All carbonates of the alkali group are thermally stable with an exception of lithium and thus readily decomposes at low temperatures. The decomposition temperatures again increase as you go down the group.