Out of the following,which pair of solutions is not a buffer solution?

The $pH$ of the blood buffer $CO_2-HCO_3^-$ is $7.4$. The ratio of conjugate base to acid is ....... $(K_a (H_2CO_3) = 4.5 \times 10^{-7})$

The $pH$ of a buffer solution decreases by $0.02$ units when $0.12 \ g$ of acetic acid is added to $250 \ mL$ of a buffer solution of acetic acid and potassium acetate at $27^{\circ} C$. The buffer capacity of the solution is

What is the value of $pOH$ if a buffer solution is prepared by mixing equal volumes of $0.4 \ M$ $NH_4OH$ and $0.5 \ M$ $NH_4Cl$ solutions? $(pK_b = 4.730)$

Assertion : Buffer system of carbonic acid and sodium bicarbonate is used for the precipitation of hydroxides of third group elements.
Reason : It maintains the $pH$ to a constant value,about $7.4.$

$20 \ mL$ of $0.1 \ M \ H_2SO_4$ is added to $30 \ mL$ of $0.2 \ M \ NH_4OH$ solution. The $pH$ of the resultant mixture is [$pK_b$ of $NH_4OH = 4.7$].