By adding a strong acid to the buffer solution,the $pH$ of the buffer solution

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

For a buffer solution containing equal concentrations of $B^-$ and $HB$,where the $K_b$ of $B$ is $10^{-10}$,what is the $pH$ of the buffer solution?

$A$ buffer solution is a mixture of

$1 \times 10^{-3} \ \text{mole}$ of $HCl$ is added to a buffer solution made up of $0.01 \ \text{M}$ acetic acid and $0.01 \ \text{M}$ sodium acetate. The final $pH$ of the buffer will be (given, $pK_{a}$ of acetic acid is $4.75$ at $25^{\circ} \text{C}$)

If $50 \ mL$ of $0.2 \ M \ KOH$ is added to $40 \ mL$ of $0.5 \ M \ HCOOH,$ the $pH$ of the resulting solution is $(K_a = 1.8 \times 10^{-4})$