An acidic buffer solution is prepared by mixing a proportionate quantity of

Given below are two statements:
Statement $I$: $A$ buffer solution is the mixture of a salt and an acid or a base mixed in any particular quantities.
Statement $II$: Blood is a naturally occurring buffer solution whose $pH$ is maintained by $H_2CO_3 / HCO_3^{-}$ concentrations.
In the light of the above statements,choose the correct answer from the options given below.

When $1.0 \ mL$ of dilute $HCl$ acid is added to $100 \ mL$ of a buffer solution with $pH = 4.0$,what will be the resulting $pH$ of the solution?

$A$ solution is prepared by mixing $0.01 \ mol$ each of $H_2CO_3$,$NaHCO_3$,$Na_2CO_3$,and $NaOH$ in $100 \ mL$ of water. The $pH$ of the resulting solution is. . . . . . .
[Given : $pK_{a1}$ and $pK_{a2}$ of $H_2CO_3$ are $6.37$ and $10.32$,respectively; $\log 2=0.30$ ]

$20 \ mL$ of $0.1 \ M$ $NaOH$ is added to $50 \ mL$ of $0.1 \ M$ acetic acid solution. The $pH$ of the resulting solution is $....... \times 10^{-2}$ (Nearest integer).
Given: $pKa$ $(CH_3COOH)$ $= 4.76$,$\log 2 = 0.30$,$\log 3 = 0.48$.

Which of the following mixtures will produce a buffer solution with $pH = 9$?