Which of the following is a buffer solution?

The $pH$ of blood in the human body is maintained by $CO_2$ and $H_2CO_3$. What is this process called?

$0.1 \ M, 60 \ mL$ $NH_4Cl$ $(K_h = 10^{-9})$ is titrated against $0.2 \ M$ $NaOH$. What is the $pH$ at the $\frac{1}{3}$ equivalence point?

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:

If a small amount of sodium acetate is added to an equimolar mixture of sodium acetate and acetic acid,the $pH$ of the resulting solution will .......

The ratio of volumes of $CH_3COOH$ $0.1 \ N$ to $CH_3COONa$ $0.1 \ N$ required to prepare a buffer solution of $pH$ $5.74$ is (Given, $pK_a$ of $CH_3COOH$ is $4.74$)