Select the correct statment(s):

Explanation:

Option A: In purification of silver, zinc is used as reducing agent not the oxidizing agent as per the equation given below:

$A{g}_{2}S+NaCN\to Na[Ag(C{N}_2]+N{a}_{2}S$

Now, when zinc is used in purification of silver 

$Na[Ag(C{N}_2]+Zn\to N{a}_2[Zn(C{N}_4]+Ag$

oxidation number of Zn changes from $0$ to $+2$

Hence it is clear that zinc is oxidised so it is used as reducing agent in purification of silver, so this option is wrong.

Option B:In the froth-flotation process, pine oil functions as a frothing agent or foaming agent. Other frothing agents include eucalyptus oil, xanthates or fatty acids. The frothing agent forms a foam (or froth) with air. Hence this is correct option.

Option C: 

Froth floatation process used for the concentration of sulphide ore uses sodium ethly xanthate, $C_2{H}_{5}OC{S}_{2}Na$ as collector.
Other examples of collector include ethyl xanthate and potassium ethyl xanthate. They attach themselves to the grains of mineral by polar groups so that minerals become water repellent and pass on into the froth .Option C is correct

 Option D: 

Froth flotation process is used to concentrate sulphide ores, but in  copper carbonate or lead sulphate we can use forth floatation method by using an activator. Option D is correct

Hence Option B, C, and D are correct answer