Explanation

(c)
Silver metal is obtained by $Mac$-Arthur Forrest process which is called cyanide process. The concentrated ore of argentite is treated with dilute NaCN solution and a current of $O_{2}$ is continuously passed. Silver sulphide goes into solution in the form of soluble complex sodium argentocyanide.
$2{Ag}_{2}S + 8NaCN + O_{2} + 2H_{2}O \longrightarrow$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 4Na\lbrack Ag(CN)_{2}\rbrack + 4NaOH + 2S$
The soluble complex is treated with zinc dust, when silver gets precipitated.
$2Na\lbrack Ag(CN)_{2}\rbrack + Zn \longrightarrow {Na}_{2}\lbrack Zn(CN)_{4} + 2Ag \downarrow$