IIT-JEE 2012 · Paper 2, Chemistry question 22

Leaching. Argentite is dissolved in NaCN with air blown through:
$$\ce{Ag2S + 4CN- <=> 2[Ag(CN)2]- + S^{2-}}.$$
This equilibrium lies to the left by itself; $\ce{O2}$ oxidises the sulphide ion (to sulphate), pulling it to completion. So $\ce{O2}$ is the oxidising agent.

Recovery. Zinc dust displaces silver from the complex:
$$\ce{2[Ag(CN)2]- + Zn -> [Zn(CN)4]^{2-} + 2Ag}.$$

Oxidising agent $\ce{O2}$, reducing agent Zn.

(B)