We have two redox reactions, one spontaneous and one non-spontaneous. Redox reactions are those in which the oxidation states of the reactants change and we can identify two types of compounds, the oxidizing agent and the reducing agent.
The oxidizing agent is one that oxidizes other substances, lowers their oxidation state and is reduced.
In the case of reactions, the oxidizing agent will be A for the first reaction and second reaction. Since A reduces its oxidation state, it goes from a 0 state to a -1 state.
But we must keep in mind that the second reaction is not spontaneous. In other words, in this case, the opposite reaction will be more important, and product C will be a stronger oxidizing agent than A.
So, the answer will be: C