In the leaching of ${\mathrm{Ag}}_2\mathrm{S}$ with $\mathrm{NaCN}$ a stream of air is also passed. It is because of: 

In the leaching of  ${\mathrm{Ag}}_2\mathrm{S}$ with $\mathrm{NaCN}$ , a stream of air is also passed. It is because of the reversible nature of  the reaction between  ${\mathrm{Ag}}_2\mathrm{S}$  and $\mathrm{NaCN}$ and to oxidise  ${\mathrm{Na}}_2\mathrm{S}$formed into ${\mathrm{Na}}_2{\mathrm{SO}}_4$   and sulphur.

Silver sulphide and sodium cyanide can react in a unidirectional manner.

Sodium sulphide is converted to sodium sulphate when a passage of air passes across it.

Hence, option C is correct.