Answer
(a) Acid
(b) Base
(c) Both
(d) Both
Work Step by Step
Acid definition by Bronsted: Substance capable of donating a proton.
(a) $HI$ can donate a proton, turning into $I^-$;
Base definition by Bronsted: Substance with the capacity of receiving one proton
(b) $C_2H_3O_2^-$ can receive a proton, turning into $C_2H_3O_2H$.
(c) $H_2PO_4^-$ can donate a proton, turning into $HPO_4^{2-}$, and it can receive a proton, turning into $H_3PO_4$
(d) $HSO_4^-$ can donate a proton, turning into $SO_4^{2-}$, and it can receive a proton, turning into $H_2SO_4$
