Which of the following is NOT aromatic?
 

The answer to the following question is given as choice C. But I think that choice D could also be a valid answer because the Carbon with the plus sign is not sp2 hybridized. Is my reasoning correct?

Aromaticity rules
 

Please remember the three rules for aromaticity:
(4n+2) pi electrons
Continuos conjugation
plane molecule

But...
 

I thought there was a rule that also said that all the atoms in the ring must be sp2 hybridized.

Is that not correct? If it is, then is it due to the plus charge "moving around" the ring if you will due to resonance, and therefore ensuring all the carbons will be sp2 hybridized?

but to but...
 

OK, continuos cojugation involve sp2 hybridization.. Let's analyse :
a: 6 pi electrons (=4x1+2)
b: 2 pi electrons (=4x0+2)
c: 4 pi electrons (non aromatic)
d: 6 pi electrons
e: 6 pi electrons

For the d structure: we have 6 pi electrons in the benzene ring. The missing electron is from the sp2 orbital and is not participate to the continuos conjugation. In my opinion, such a cation is not stable and that is just a theoretical problem. More on the problem of aromaticity you will find working on Huckel equations.

I had been this question during my studies. But still couldn't understand how to differentiate whether they are aromatic or not. Is there any principle or law to distinguish them?

Here's the deal with D, I believe. D has a positive charge, so it's short 2 electrons. Therefore, in D, a hydrogen left and took the electron pair with it.

It still has its other electrons involved in the conjugated pi system, so these are unaffected. So it retains its aromaticity.

Does that make sense?

But thats exactly what would make it non-aromatic. By losing those two electrons it will behave differently, similar to B. I am not sure of the answer, but I think it is one of those abnormal qualities of benzene that allows it to still be aromatic?

Those of you who posted after adrian don't seem to have understood xyr explanation. Aromatic systems always have 4n+2 pi electrons, or 2n+1 pi bonds, with n being zero or a positive integer. Because C has 4 pi electrons (2 pi bonds) it is not aromatic;

4(0)+2 = 2
4(1)+2 = 6

There's no way to get 4 from 4n+2 if n's an integer. All the others have the proper number of pi electrons/bonds.