Answer
Both $(c)$ and $(d)$ are capable of acting as a buffer.
Work Step by Step
(a) $KCl/HCl$
These compounds can't form a buffer, because $HCl$ is totally dissociated in water; therefore, we don't have an equilibrium in the solution.
Since $Cl^-$ isn't a significant base, these compounds are not capable of neutralizing an added acid.
(b) $KHSO_4/H_2SO_4$
These compounds can't form a buffer, because $H_2SO_4$ is totally dissociated in water; therefore, we don't have an equilibrium in the solution.
Since $HS{O_4}^-$ isn't a significant base, these compounds are not capable of neutralizing an added acid.
(c) and (d):
They both have a weak acids, $H_2PO_4$ and $HNO_2$, and a weak base, that is the conjugate pair of the acid: $HP{O_4}^-$ and $NO_2$.
