The first I.E  of Na , Mg , Al,and are in the order :

In a period from left to right ionization enthalpy increases. For Al (3s²3p¹ ), electron has to be removed from partially filled 3p orbital whereas in Mg(3s²), electron has to be removed from stable fully filled 3s orbital.
Removal of an electron from stable, fully filled orbital requires more energy than removal of electron from partially filled orbital. Thus, ionization enthalpy for Mg is greater than ionization enthalpy for Al. So, the correct order of first ionization enthalpies is: Na<Mg>Al<Si