Factors Affecting Ionisation potential
1)The values of ionisation potential of an element decreases as its atomic radius increases.
2)Ionisation potential ∝ Effective nuclear charge (Zeff)
3)Ionisation potential is inversely proportional to shielding or screening effect
4)Stability of half−filled and fully-filled orbitals
In the above options, lithium has smallest ionic radius, hence highest ionization potential.