The correct option is B ZnS forms soluble complex Na2[Zn(CN)4] while PbS forms froth
Both ZnS and PbS forms froth with collector in the froth flotation process.
Addition of NaCN (depressant), selectively prevents ZnS from coming to the froth by forming a soluble complex Na2[Zn(CN)4] but allows PbS to come with the froth. So, option (b) is the correct option.