The first ionisation enthalpies of Na, Mg, Al and Si are in the order:
(i) Na < Mg > Al < Si
(ii) Na > Mg > Al > Si
(iii) Na < Mg < Al < Si
(iv) Na > Mg > Al < Si
The first ionisation enthalpies of Na, Mg, Al and Si are in the order:
(i) Na < Mg > Al < Si
(ii) Na > Mg > Al > Si
(iii) Na < Mg < Al < Si
(iv) Na > Mg > Al < Si

Option (i) is the answer.

Ionization enthalpy is the enthalpy change associated with the loss of the first electron from an isolated gaseous atom in its ground state. As we move across the period, nuclear charge increases, atomic size decreases, and thus ionization enthalpy increases.

The electrons in the partially filled 3p orbital of Al and in the fully filled 3s orbital of Mg must be removed to find the ionization enthalpy. The energy required to remove an electron from a stable fully filled orbital is more than that required to remove an electron from a partially filled orbital. As a result, the ionization enthalpy of Mg is larger than that of Al. As a result, the correct order of the first ionization enthalpies is as follows: Na<Mg>Al<Si