Assertion: On moving down the group, the ionization enthalpy decreases.
Reason: With decrease in size of atom, the force of attraction between the nucleus and valence electron decreases.
(A) Both Assertion and Reason are correct and reason is the correct explanation of the assertion.
(B) Both Assertion and Reason are correct and reason is not the correct explanation of the assertion.
(C) Assertion is correct but reason is incorrect.
(D) Assertion is incorrect but reason is correct.

Hint :Ionisation enthalpy is the amount of energy required by the isolated gaseous atom to remove electrons from its valence shell in ground state. The removal of electrons from the valence shell results in the formation of cations.

Complete Step By Step Answer:
To solve this question, we have to give our focus on the periodic table.
Since, we know that the ionisation enthalpy depends on the size of the atom.
So, according to the rules, in the periodic table when we move down the table or go from left to right, then the size of the atom decreases.
Hence, we can say that when we go down the group the ionisation energy decreases. Therefore this indicates that our assertion is true.
Now, let’s look at the reason for the same. So when we go down the group, the atomic size of the atom increases and also the distance between the nucleus and the valence shell increases. This results in the increase of shielding effect and thus decrease in the effective nuclear charge.
This less interaction between the nucleus and the valence shell makes the electrons to be removed easily from the valence shell and hence low ionisation energy is required.
Thus, on the basis of the above discussion, we can say that our Assertion is correct but Reason is incorrect.
Therefore option C is correct.

Note :
Keep in mind that atoms can have first, second, third and so on ionisation energies. Also the subsequent ionisation enthalpy is always greater than the previous one because in the first ionisation enthalpy we have to remove the electron from the atom but in second ionisation enthalpy we have to remove the electron from the ion or cation.