The correct order of ionization potential [IP₁] among the IIIA group elements if

On moving down the group, the ionization enthalpy decreases. This is true for B and Al. 
     
The ionization enthalpy of Ga is unexpectedly higher than that of Al. Ga contains 10d electrons in inner shell which are less penetrating. Their shielding is less effective than that of s and p electrons. The outer electron is held fairly strongly by the nucleus. The ionization enthalpy increases slightly.

A similar increase is observed from In to Tl due to presence of 14f electrons in the inner shell of Tl which have poor shielding effect.