Which of the following is not a reducing agent?

Which oxide among the following cannot act as a reducing agent?

Identify the reducing agent in the following chemical equations:
$4Ag(s) + 8CN^{-}(aq) + 2H_{2}O(aq) + O_{2}(g) \rightarrow 4[Ag(CN)_{2}]^{-}(aq) + 4OH^{-}(aq)$
$2[Ag(CN)_{2}]^{-}(aq) + Zn(s) \rightarrow [Zn(CN)_{4}]^{2-}(aq) + 2Ag(s)$

Which of the following mixtures is chromic acid?

Which acts as an oxidising agent in the following reaction?
$2H_2O_{(l)} + 2Na_{(s)} \to 2NaOH_{(aq)} + H_{2_{(g)}}$

In the following reaction,how does sulfuric acid act?
${H_2}SO_4 \to SO_2$