The correct option is C NaCN reacts with ZnS to form Na2[Zn(CN)4]
A depressant helps in separating two sulphide ores by selective prevention of froth formation by one of the ores allowing the other one to form froth.
ZnS forms a soluble complex Na2[Zn(CN)4] with NaCN while PbS does not and goes up with the froth.
4NaCN+ZnS→Na2[Zn(CN)4]+Na2S