Toluene, when treated with ${\mathrm{Br}}_2/\mathrm{Fe}$  gives p-bromo toluene as the major product because ${\mathrm{CH}}_3$ group:

${\mathrm{H}}_2{\mathrm{SO}}_4$and ${\mathrm{HClO}}_4$ are stronger acid than ${\mathrm{HNO}}_3$ so that ${\mathrm{HNO}}_3$ acts as base with them.

(a) and (b) 

(d) ${\mathrm{HNO}}_2$ first gives 

which is oxidised by ${\mathrm{HNO}}_3$ to give 

and ${\mathrm{HNO}}_2.$ So, it can also be used as catalyst.