Which of the following mixtures in water acts as a buffer?

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

$A$ solution of $H_3BO_3$ and borax is known as:

When $500 \, mL$ of $0.2 \, M$ acetic acid is added to $500 \, mL$ of $0.30 \, M$ sodium acetate solution,what is the $pH$ of the resulting solution? (Given: Dissociation constant of acetic acid $K_a = 1.5 \times 10^{-5}$)

What would be the $pH$ of a solution obtained by mixing $5 \ g$ of acetic acid and $7.5 \ g$ of sodium acetate and making the volume equal to $500 \ mL$? $(K_a = 1.75 \times 10^{-5}, pK_a = 4.76)$

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