Which from following mixtures in water acts as a buffer?

Calculate the solubility in $\mathrm{mol} \mathrm{dm}^{-3}$ of sparingly soluble salt BA if its solubility product $4.9 \times 10^{-13}$ at same temperature.

An aqueous solution of strong monoacidic base is of $1 \times 10^{-4} \mathrm{M}$. What is the value of pH at $25^{\circ} \mathrm{C}$ ?

Acidic buffer solution is prepared by mixing proportionate quantity of