Heating benzene in a large excess of $80\% \, D_2SO_4$ in $D_2O$ results in what product?

Which of the following are aromatic compounds?

Among the following compounds,the order of decreasing reactivity towards electrophilic substitution is:

Amongst the following compounds,the one which is most easily sulphonated is:

The best reagent for nuclear iodination of aromatic compounds is:

Consider the following sequence of reactions. In $Z$,the number of $sp^3$ carbons is $a$ and $sp^2$ carbons is $b$. The value of $(a+b)$ is:
$CH_3CH_2CH_2CH_2Br$ $\xrightarrow[\text{dry ether}]{Na} X$ $\xrightarrow[\text{10-20 atm}]{\text{Cr}_2\text{O}_3 / 773 \text{ K}} Y$ $\xrightarrow[\text{anhy. AlCl}_3]{\text{CH}_3\text{COCl}} Z$