In nitration of benzene using conc. $H_{2}S{O}_4$ and conc. $HN{O}_3,$ the role of nitric acid is as

The correct option is B a base
Nitration is electrophilic substitution reaction. In its first step $HN{O}_3$ takes protons from sulphuric acid and then forms $-N{O}_2^{+}$ as an attacking electrophile for the reaction. So, here $HN{O}_3$ acts as a base.

Mechanism of reaction is shown as:

So, (b) is correct.