The element having the highest first ionization enthalpy is

The first ionisation energy is the energy needed to remove the outermost, or highest energy, electron from a neutral atom in the gas phase. 

The ionisation energy decreases from top to bottom in groups, and increases from left to right across a period. Nitrogen and carbon belong to second period elements, hence they have more ionisation energy than the other two elements.

It is highest for nitrogen because nitrogen is a more stable half-filled electronic configuration.

$Al < Si < C < N$; ${IE}_{1}$ order.

Hence option C is the answer.