Assertion: The carbonate of lithium decomposes easily on heating to form lithium oxide and CO2.

Reason: Lithium being very small in size polarises large carbonate ions leading to the formation of more stable Li2O and CO2.

1.  Both assertion and reason are true and the reason is the correct explanation of assertion.

2.  Both assertion and reason are true and the reason is not the correct explanation of assertion.

3.  Assertion is true but the reason is false.

4.  Assertion is false but the reason is true.

Hint: The thermal stability of carbonates increases down the group.
 
The thermal stability of carbonates increases down the group. Hence, Li2CO3 is the least stable. Due to small size of Li+, strong polarising power distorts the electron cloud of CO32- ion.
The high lattice energy of Li2O than Li2CO3 also favors the decomposition of Li2CO3.