Explanation:

Ionization energy is the energy required to remove an electron from an atom or ion in the gaseous state. The third ionization energy is the energy required to remove the third electron from a positively charged ion.

In order to understand why the third ionization energy is maximum for nitrogen, we need to look at the electronic configuration of the elements given in the options.

a) Boron: Electronic configuration of Boron is 1s²2s²2p¹. It has three valence electrons.

b) Phosphorus: Electronic configuration of Phosphorus is 1s²2s²2p⁶3s²3p³. It has five valence electrons.

c) Aluminium: Electronic configuration of Aluminium is 1s²2s²2p⁶3s²3p¹. It has three valence electrons.

d) Nitrogen: Electronic configuration of Nitrogen is 1s²2s²2p³. It has five valence electrons.

Now, let's look at the factors that affect ionization energy:

1. Atomic radius: As the atomic radius increases, the ionization energy decreases.

2. Shielding effect: As the number of inner electrons increases, the ionization energy decreases.

3. Nuclear charge: As the nuclear charge increases, the ionization energy increases.

Based on these factors, we can see that Nitrogen has the highest third ionization energy because:

1. Nitrogen has a smaller atomic radius than Phosphorus, Aluminium, and Boron.

2. Nitrogen has a higher nuclear charge than Phosphorus, Aluminium, and Boron.

3. Nitrogen has a half-filled p subshell, which makes it more stable than the other elements.

Therefore, the correct answer is option 'D' Nitrogen.