In the thermite reaction $3\mathrm{Fe}_3\mathrm{O}_4(s) + 8\mathrm{Al}(s) \rightarrow 9\mathrm{Fe}(s) + 4\mathrm{Al}_2\mathrm{O}_3(s)$, which species undergoes reduction?

1. Apply the classical definition: reduction = removal of oxygen (or addition of hydrogen). 2. In $\mathrm{Fe}_3\mathrm{O}_4$, iron starts in mixed +2/+3 oxidation states; in product Fe(s) iron is 0. So Fe goes DOWN in oxidation number — reduction. 3. Aluminium goes from 0 in Al(s) to +3 in $\mathrm{Al}_2\mathrm{O}_3$, an INCREASE — oxidation, not reduction. So options A and C are wrong. 4. $\mathrm{Al}_2\mathrm{O}_3$ is a PRODUCT and is the oxidised form of Al; it is not undergoing further reduction in this reaction (option D wrong). _Source: NCERT Class 11 Chemistry Part 2, Ch 7, Problem 7.1 (ii), p. 1–2._