Which of the following will not function as a buffer solution?

Calculate $[H^{+}]$ of $0.2 \ M$ $HCN$ in $1 \ M$ $KCN$ solution. Given $K_{a(HCN)} = 4 \times 10^{-10}$.

$40 \ mL$ of $0.1 \ M$ ammonia solution is mixed with $20 \ mL$ of $0.1 \ M \ HCl$. What is the $pH$ of the mixture? ($pK_b$ of ammonia solution is $4.74$).

Assertion : In a titration of weak acid and $NaOH$,the $pH$ at half equivalence point is $pK_a$.
Reason : At half equivalence point,it forms an acidic buffer and the buffer capacity is maximum where $[acid] = [salt]$.

Which of the following mixtures will form a buffer solution?

$A$ buffer solution is a mixture of