$A$ buffer solution can be prepared by mixing which of the following?

Explain the preparation of an acidic buffer solution with an example.

$A$ solution of $0.1 \ mol$ of $CH_3NH_2$ $(K_b = 5 \times 10^{-4})$ and $0.08 \ mol$ of $HCl$ is diluted to $1 \ L$. The $pOH$ of the resulting solution is $(\log 1.25 = 0.1)$.

Which of the following is a buffer solution?

$A$ buffer solution is prepared by mixing $10 \ mL$ of $1.0 \ M$ acetic acid and $20 \ mL$ of $0.5 \ M$ sodium acetate and then diluted to $100 \ mL$ with distilled water. The $pH$ of the buffer solution is ($pK_a$ of acetic acid is $4.76$).

$A$ buffer solution is prepared by mixing $0.1 \ M \ HCN$ and $0.2 \ M \ NaCN$. What is the $pH$ of the buffer solution if $pKa$ of $HCN$ is $9.3$? $(\log 2 = 0.3010)$