Answer
$(a)$, $(b)$ and $(c)$ are capable of forming buffers.
Work Step by Step
1. To be capable of forming a buffer, the solution needs:
- A weak acid.
- Its conjugate base.
(a) Weak acid: $HCN$; Conjugate base: $CN^-$;
(b) Weak acid: $HS{O_4}^-$; Conjugate base: $S{O_4}^{2-}$
(c) Weak acid: $N{H_4}^+$; Conjugate base: $NH_3$
(d) Weak acid: Missing ($HI$ is a strong acid).
