The $pH$ of a buffer solution made by mixing $25\;ml$ of $\;0.02\;M\;NH_4OH$ and $25\;ml$ of $\;0.2\;M\;NH_4Cl$ at $25^{\circ}C$ is : $(pK_b\;of\;NH_4OH=4.8)$

Question

$$(a)\;5.8 \quad (b)\;8.2 \quad (c)\;4.8 \quad (d)\;3.8$$