Credit for the ring structure of benzene goes to

Which of the following is an example of a non-benzenoid aromatic compound?

What type of difference is observed in the hybridization of carbon in benzene during nitration and the formation of the $\sigma$-complex?

The species responsible for nitration and sulphonation by nitric acid/conc. $H_2SO_4$ and fuming $H_2SO_4$ are :

From the following compounds, choose the one which is not aromatic.

All the hydrocarbons shown are very weak acids. One,however,is far more acidic than the others. Which one is the strongest acid?