In the following reactions, ZnO is respectively acting as a/an:
(a) ZnO + Na₂O → Na₂ZnO₂
(b) ZnO + CO₂ → ZnCO₃

In reaction (a), ZnO reacts with Na₂O (a basic oxide) to form Na₂ZnO₂. ZnO acts as an acidic oxide because it accepts O²⁻ ions from the basic oxide Na₂O. In reaction (b), ZnO reacts with CO₂ (an acidic oxide) to form ZnCO₃. ZnO acts as a basic oxide because it donates O²⁻ ions to the acidic oxide CO₂.
Therefore, ZnO acts as an acid and a base, respectively. The correct answer is (c) acid and base.