Which of the following solutions cannot act as a buffer?

$A$ certain buffer solution contains equal concentrations of $3.9 \times 10^{-5} \ M$ $X^{-}$ and $HX$. The $K_b$ for $X^{-}$ is $10^{-10}$. The $pH$ of the buffer is:

$20 \ mL$ of $0.1 \ (N)$ acetic acid is mixed with $10 \ mL$ of $0.1 \ (N)$ solution of $NaOH$. The $pH$ of the resulting solution is ($pK_a$ of acetic acid is $4.74$). (in $.74$)

The $pH$ of a solution at $25\,^oC$ containing $0.10\,M$ sodium acetate and $0.03\,M$ acetic acid is ($pK_a$ for $CH_3COOH = 4.57$)

When a buffer solution of sodium acetate and acetic acid is diluted with water,

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 $(K_a = 1.8 \times 10^{-4})$