Which of the following pairs constitutes a buffer?

The dissociation constant of $HCN$ is $5 \times 10^{-10}$. The $pH$ of the solution prepared by mixing $1.5 \, mole$ of $HCN$ and $0.15 \, mole$ of $KCN$ in water and making up the total volume to $0.5 \, dm^3$ is (in $.302$)

The $pK_a$ of a weak acid is $4.8$. What should be the ratio of $\frac{[\text{acid}]}{[\text{salt}]}$, if a buffer of $pH = 5.8$ is required?

What is the ratio of $pH$ for a solution containing $1 \ mol$ $CH_3COONa$ + $0.5 \ mol$ $HCl$ per liter and another solution containing $1 \ mol$ $CH_3COONa$ + $1 \ mol$ $CH_3COOH$ per liter?

Which of the following does not form a buffer solution?

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