In the reaction,
$$
\mathrm{HC}_{2} \mathrm{O}_{4}^{-}+\mathrm{PO}_{4}^{3-} \rightleftharpoons \mathrm{HPO}_{4}^{2-}+\mathrm{C}_{2} \mathrm{O}_{4}^{2}
$$
the Bronsted bases are :
(a) $\mathrm{HC}_{2} \mathrm{O}_{4}^{-}$
(b) $\mathrm{PO}_{4}^{3-}$
(c) $\mathrm{HPO}_{4}^{2-}$
(d) $\mathrm{C}_{2} \mathrm{O}_{4}^{2-}$