b
Proton donor is acids and proton acceptor is bases. Conc. $\mathrm{H}_{2} \mathrm{SO}_{4}$ and conc. HNO $_{3}$ react in the following manner:
$\mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{H}_{2} \mathrm{NO}_{3}^{+}+\mathrm{HSO}_{4}^{-}$
$\mathrm{H}_{2} \mathrm{NO}_{3}^{+} \rightarrow \mathrm{NO}_{2}^{+}+\mathrm{H}_{2} \mathrm{O}$
Hence, in this reaction HNO $_{3}$ acts as a base and $\mathrm{H}_{2} \mathrm{SO}_{4}$ as an acid.