Here it says in this example, why does the pyridine ring not undergo nitration at c₄? Well, when we worked out all the resonance structures of our pyridine, we saw that the c₃ position was the most preferred. That's because we never created the unstable positive nitrogen intermediate. All the positive charges were distributed on the carbon atoms, which are less electronegative. So, if we take a look here, the one that most fits in with this description, being the furthest from the nitrogen, no.

The reaction intermediate has an electron-deficient nitrogen with a positive charge. Yes. This is the answer. The pyridine ring is more easily nitrated at the nitrogen atom instead of carbon 4? No.

The c₄ position gets protonated under the acidic conditions of nitration. Again, no. It has to do with the formation of that very unstable positive nitrogen intermediate. That is not ideal for positions c₂ or c₄. So that's why we do nitration at c₃ instead.