Which of the following solutions can act as a buffer?

In a mixture of acetic acid and sodium acetate,the ratio of the concentration of the salt to the acid is increased ten times. Then the $pH$ of the solution:

Which of the following solutions will act 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 $H_{2}CO_{3})$.

The $pH$ of blood is maintained by a proper balance of $H_2CO_3$ and $NaHCO_3$ concentrations. What volume of $5 \, M \, NaHCO_3$ solution should be mixed with $10 \, mL$ of $2 \, M \, H_2CO_3$ to maintain a $pH$ of $7.4$ (in $, mL$)? (Given: $K_a$ for $H_2CO_3 = 7.8 \times 10^{-7}$)

For a weak acid $(HA)$,$pK_a = 4.5$. The $pOH$ of a buffer solution of $HA$ in which $50\%$ of the acid is ionized is: