Which one of the following pairs of solutions is not an acidic buffer?

An acidic buffer is obtained on mixing

Calculate the $pH$ of a buffer solution containing $0.35 \ M$ weak acid and $0.70 \ M$ of its salt with a strong base if $pK_{a}$ is $4.56$.

$A$ solution contains $0.1 \ mol$ of acetic acid and $0.1 \ mol$ of sodium acetate in $500 \ mL$ of solution. If the $pH$ of the solution is $4.74$,find the ionization constant $(K_a)$ of acetic acid.

The $pK_b$ of $CN^{\Theta}$ is $4.7$. The $pH$ of a solution prepared by mixing $2.5 \ mol$ of $KCN$ and $2.5 \ mol$ of $HCN$ in water and making the total volume up to $500 \ mL$ is:

$A$ buffer solution is prepared by mixing $0.2 \ M \ NH_4OH$ and $1 \ M \ NH_4Cl$. What is the $pH$ value of the buffer solution? (Given $pK_b = 4.744$)