Answer
(A) $H{CO_3}^-$ and (C) $H_2O$
Work Step by Step
(a) $HC{O_3}^-$ can act like an acid, and turn into $C{O_3}^{2-}$, or can act like a base, and turn into $H_2CO_3$
(b) $CN^-$ is a base, but it can't act like an acid.
(c) $H_2O$ can act like an acid, and turn into $OH^-$, or can act like a base, and turn into $H_3O^+$.
(d) $H_2CO_3$ is an acid, but it can't act like a stable base.
