Dilute solution of sodium cyanide is used in the ore dressing of silver and gold. This process is familiarly called

• Levigation is the process of grinding an insoluble substance to a fine powder, while wet.
•  Liquiation is a technique for separating constitutes of an ore , a metal or an alloy by partial melting. Leaching involves the treatment of ore with leaching agents (like $\mathrm{NaOH},\mathrm{NaCN},\mathrm{KCN}$ or other reagents) due to which ore becomes soluble and their impurities remain insoluble.
• Poling  is a refining process which is used for the purification of copper which contains copper oxide as an impurity.

Therefore, From the above explanation option(C) is the correct answer.