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; IE1 order.
Hence option C is the answer.