Which of the following buffer solutions will have the highest acidic character?

$A$ buffer solution is prepared by mixing $10 \ mL$ of $1.0 \ M$ acetic acid and $20 \ mL$ of $0.5 \ M$ sodium acetate and then diluted to $100 \ mL$ with distilled water. The $pH$ of the buffer solution is ($pK_a$ of acetic acid is $4.76$).

$50 \ mL$ of $0.02 \ M$ $NaOH$ solution is mixed with $50 \ mL$ of $0.06 \ M$ acetic acid solution. The $pH$ of the resulting solution is $......$ ($pK_a$ of acetic acid is $4.76$,$\log 2 = 0.30$).

An acidic buffer is obtained on mixing

Which among the following pairs constitutes a buffer?

The dissociation constant of acetic acid is $x \times 10^{-5}$. When $25 \ mL$ of $0.2 \ M \ CH_3COONa$ solution is mixed with $25 \ mL$ of $0.02 \ M \ CH_3COOH$ solution,the $pH$ of the resultant solution is found to be equal to $5$. The value of $x$ is $..........$.