Identify the nature of $ZnO$

Zinc oxide, $ZnO$, is an amphoteric oxide. This means it can behave both as an acid and as a base, depending on the reacting substance.

Analyze reaction (a)

$ZnO + Na_2O \to Na_2ZnO_2$

• $Na_2O$ is a basic oxide.
• When an amphoteric oxide reacts with a basic oxide, it behaves as an acid.
• Here, $ZnO$ reacts with basic $Na_2O$ to form sodium zincate.

Therefore, in reaction (a), $ZnO$ acts as an acid.

Analyze reaction (b)

$ZnO + CO_2 \to ZnCO_3$

• $CO_2$ is an acidic oxide.
• When an amphoteric oxide reacts with an acidic oxide, it behaves as a base.
• Here, $ZnO$ reacts with acidic $CO_2$ to form zinc carbonate.

Therefore, in reaction (b), $ZnO$ acts as a base.

Final conclusion

• In (a): acid
• In (b): base

So the correct option is:

$\boxed{\text{C: acid and base}}$

Comparison with stored correct answer

Stored correct answer: $C$

Our derived answer is also $C$, so they match.