Explanation

(d)
Impurities of Cu and Ag from gold are removed by boiling impure gold with conc. $H_{2}{SO}_{4}\$ and also by electrolytic method.
$Cu + 2H_{2}{SO}_{4}\overset{Heat}{\rightarrow}{CuSO}_{4} + {SO}_{2} + 2H_{2}O$
$2Ag + 2H_{2}{SO}_{4}\overset{Heat}{\rightarrow}{Ag}_{2}{SO}_{4} + {SO}_{2} + 2H_{2}O$
This method is called parting. Conc. ${HNO}_{3}$ can also be used for this purpose.