Which of the following is a buffer solution?

The principal buffer present in human blood is:

An aqueous buffer is prepared by adding $100 \, mL$ of $0.1 \, mol \, L^{-1}$ acetic acid to $50 \, mL$ of $0.2 \, mol \, L^{-1}$ of sodium acetate. If $pK_a$ of acetic acid is $4.76$,the $pH$ of the buffer is

$A$ weak acid,$HA$,is found to be $10\%$ ionized in $0.01\ M$ aqueous solution. Calculate the $pH$ of a solution which is $0.1\ M$ in $HA$ and $0.05\ M$ in $NaA$.

$A$ buffer solution contains $0.1 \ M$ of acetic acid and $0.1 \ M$ of sodium acetate. What will be its $pH$,if $pK_a$ of acetic acid is $4.75$?

The $pH$ of an acidic buffer mixture is