Which of the following buffer solutions will have the highest acidic character?

$A$ 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 $(\log 1.25 = 0.1)$.

$50 \ mL$ of $0.02 \ M$ $NaHSO_4$ is mixed with $50 \ mL$ of $0.02 \ M$ $Na_2SO_4$. Calculate the $pH$ of the resulting solution. $[pK_{a2} (H_2SO_4) = 2]$

Which of the following solutions does not act as a buffer?

$A$ solution contains $0.2 \ M \ NH_4OH$ and $0.2 \ M \ NH_4Cl$. If $1.0 \ mL$ of $0.001 \ M \ HCl$ is added,what will be the $[OH^-]$ of the resulting solution? [Given: $K_b = 2 \times 10^{-5}$]

Which of the following mixtures of solutions can function as a buffer solution?