Which of the following is not an acidic buffer?

What is the $pH$ of a buffer solution prepared by mixing $0.01 \ M$ weak acid and $0.02 \ M$ salt of weak acid with a strong base? $(pK_{a} = 4.680)$

$20 \, mL$ 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 $5$.

If $50 \, mL$ of $2 \, N$ acetic acid is mixed with $10 \, mL$ of $1 \, N$ sodium acetate solution,what will be the approximate $pH$ of the resulting solution? (Given: $K_a = 10^{-5}$)

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

The $pH$ of a buffer solution decreases by $0.02$ units when $0.12 \ g$ of acetic acid is added to $250 \ mL$ of a buffer solution of acetic acid and potassium acetate at $27^{\circ} C$. The buffer capacity of the solution is