(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
In ZnO + C → Zn + CO, carbon removes oxygen from ZnO (reducing it to Zn), so ZnO is reduced and C acts as the reducing agent.
ZnO loses oxygen → it undergoes reduction. Carbon gains oxygen → it is oxidised, acting as a reducing agent. Both statements are correct and R directly explains why A is true, making option (a) the right choice. Examiners expect students to identify the role of each substance in terms of gain/loss of oxygen.