The $pH$ of an acidic buffer mixture is

Which of the following will not function as a buffer solution?

The $[HCO_{3}^{-}]/[H_{2}CO_{3}]$ ratio in the buffer present in blood $(pH = 7.4)$ is approximately $(pK_{a} = 6.1$ for $H_{2}CO_{3})$.

$pH$ of a mixture which is $0.1 \ M$ in $CH_3COOH$ and $0.05 \ M$ in $(CH_3COO)_2Ba$ is [$pK_a$ of $CH_3COOH$ = $4.74$]

$A$ solution of formic acid is half-neutralized by the addition of a $KOH$ solution. If $K_a (HCOOH) = 2 \times 10^{-4}$,what is the $pH$ of the solution? (Given: $\log 2 = 0.3010$)

The main buffer present in our blood is.....