Explanation

(b)
${Ag}_{2}{SO}_{4}$ contains ${Ag}^{+}$ ($4d^{10}$) and is colourless. ${CuF}_{2}$ contains ${Cu}^{2 +}$ ($3d^{9}$) and is coloured due to the presence of one unpaired electron in $d$-orbital of ${Cu}^{2 +}$.
${MgF}_{2}$ contains ${Mg}^{2 +}$ and is colourless $n$/2 CuCl contains ${Cu}^{+}$ ($3d^{10}$) and is colourless.