Answer
(a) Base
(b) Base
(c) Acid
(d) Both
Work Step by Step
Acid definition by Bronsted: Substance capable of donating a proton.
(c) $NH_4^+$ can donate a proton, turning into $NH_3$;
Base definition by Bronsted: Substance with the capacity of receiving one proton
(a) $PO_4^{3-}$ can receive a proton, turning into $HPO_4^{2-}$.
(b) $ClO_2^-$ can receive a proton, turning into $HClO_2$
(d) $HCO_3^-$ can donate a proton, turning into $CO_3^{2-}$, and it can receive a proton, turning into $H_2CO_3$
