Which of the following is a buffer solution?

An acidic buffer solution can be prepared by mixing the solution of

$20 \ mL$ 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 $NH_4OH = 4.7$].

$A$ buffer solution is prepared by mixing $0.2 \ M \ NH_4OH$ and $1 \ M \ NH_4Cl$. What is the $pH$ value of the buffer solution? (Given $pK_b = 4.744$)

Find the $pH$ of a solution formed by mixing equal volumes of $0.1 \text{ M}$ sodium propionate and $0.1 \text{ M}$ propionic acid. (Given: The dissociation constant of propionic acid is $K_a = 1.3 \times 10^{-5}$)

The $pK_a$ of a weak acid $(HA)$ is $4.5$. The $pOH$ of an aqueous buffer solution of $HA$ in which $50\%$ of the acid is ionized is