The acidic and basic strength of buffer solutions remains constant because ...

Which mixture forms a buffer when dissolved in $1 \, L$ of water?

Aqueous solutions of $HNO_3$,$KOH$,$CH_3COOH$,and $CH_3COONa$ of identical concentrations are provided.
The pair$(s)$ of solutions which form a buffer upon mixing is(are):
$A$. $HNO_3$ and $CH_3COOH$
$B$. $KOH$ and $CH_3COONa$
$C$. $HNO_3$ and $CH_3COONa$
$D$. $CH_3COOH$ and $CH_3COONa$

In order to prepare a buffer solution of $pH = 5.74$,sodium acetate is added to acetic acid. If the concentration of acetic acid in the buffer is $1.0 \ M$,the concentration of sodium acetate in the buffer is $......... \ M$. (Round off to the nearest integer).
[Given: $pKa$ (acetic acid) $= 4.74$]

$A$ buffer solution is prepared with a concentration of $0.30 \ M \ NH_3$ and $0.20 \ M \ NH_4^+$. If the equilibrium constant for $NH_3$ is $K_b = 1.8 \times 10^{-5}$,what will be the $pH$ of this solution?

Find $pH$ of the resultant solution formed by the addition of $500 \ mL$ $0.1 \ M$ $Ba(OH)_2$ and $500 \ mL$ $0.6 \ M$ $NH_4Cl$ in $1 \ L$ pure $H_2O$ at $25 \ ^oC$ (Use $\log \ 2 = 0.3$,$\log \ 3 = 0.5$,$\log \ 5 = 0.7$,$K_b(NH_3) = 10^{-5}$)