Answer: B. In the blast furnace, iron(III) oxide (haematite) is reduced to iron by carbon and carbon monoxide. The carbon in coke is itself oxidised — first to CO (\(2\text{C} + \text{O}_2 \rightarrow 2\text{CO}\)), then potentially further to \(\text{CO}_2\) (\(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\)).
Carbon goes from oxidation state 0 (in coke) to \(+2\) (in CO) or \(+4\) (in \(\text{CO}_2\)) — loses electrons — oxidised.
A (\(\text{CO}_2\)) is a product, not a reactant being oxidised — and at \(+4\) it is already at C's highest common oxidation state.
C (haematite, \(\text{Fe}_2\text{O}_3\)) is reduced to Fe — the opposite of oxidation.
D (limestone, \(\text{CaCO}_3\)) decomposes to \(\text{CaO}\) and \(\text{CO}_2\) — no change in oxidation state for any element.
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