Explanation

(a)
Cyanide process is used for extraction of silver (Ag).
$4Ag + 8NaCN\ + 2H_{2}O + O_{2} \rightleftharpoons 4Na\lbrack Ag(CN)_{2}\rbrack + 4NaOH$
$2Na\lbrack Ag(CN)_{2}\rbrack + 4NaOH + Zn \longrightarrow$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {Na}_{2}{ZnO}_{2} + 2H_{2}O + 4NaCN + 2Ag$