The order of decreasing ionisation enthalpy in alkali metals is

1. Na>Li>K>Rb

2. Rb<Na<K<Li

3. Li>Na>K>Rb

4. K<Li<Na<Rb


Hint: Down the group ionization energy decreases

On moving down in the group (from Li to Cs), the ionisation energy value decreases from Li to Cs, size of the atom increases and so valence electron is less tightly held. Increased screening effect from Li to Cs also makes the removal of electron easier.