Explanation

(d)
Hydrometallurgy is based on reduction. In this process, more electropositive Zn metal is used to precipitate gold, silver etc. from their complex salt solutions.
$2K\ Au(CN)_{2} + Zn \longrightarrow \ K_{2}Zn(CN)_{4} + 2Au$
$2Na\ Ag(CN)_{2} + Zn \longrightarrow \ {Na}_{2}Zn(CN)_{4} + 2Ag$
Alkali metals or aluminium can also reduce complex salts.
$K_{2}{TiF}_{6} + 4K\ \longrightarrow 6KF + Ti$
$K_{2}{ZrF}_{6} + 2Al\ \longrightarrow 2{AlF}_{3} + 2K + Zr$