Which one of the following is not a buffer solution?

$20 \, mL$ of $0.2 \, M \, HCN$ is mixed with $10 \, mL$ of $0.2 \, M \, NaOH$. Calculate the $pH$ of the resulting mixture. The $pKa$ value of $HCN$ is $5$.

The $pH$ of a buffer solution containing $0.2 \ mol/L$ $CH_3COONa$ and $1.5 \ mol/L$ $CH_3COOH$ is ($K_a$ for acetic acid is $1.8 \times 10^{-5}$).

To prepare a buffer solution of $pH = 6$ by mixing sodium acetate and acetic acid,what should be the ratio of the concentration of salt to acid? $(K_a = 10^{-5})$

The $pH$ of a buffer solution obtained by mixing $50 \ mL$ of $0.1 \ M$ $NH_4OH$ and $25 \ mL$ of $2.0 \ M$ $NH_4Cl$ is ($pK_b$ of $NH_4OH$ is $4.8$).

Find the $pH$ of a solution formed by mixing equal volumes of $0.1 \text{ M}$ sodium propionate and $0.1 \text{ M}$ propionic acid. (Given: The dissociation constant of propionic acid is $K_a = 1.3 \times 10^{-5}$)