In the following reactions, ZnO is respectively acting as a/an,
(i) ZnO + Na$$_{2}$$O → Na$$_{2}$$ZnO$$_{2}$$
(ii) ZnO + CO$$_{2}$$ → ZnCO$$_{3}$$

We start by recalling that zinc oxide, written as $$ZnO$$, is an amphoteric oxide. An amphoteric oxide can behave either as an acid (that is, it can accept oxide ions from a base) or as a base (that is, it can donate oxide ions to an acid), depending on the other substance present.

First we examine reaction (i)

$$ZnO + Na_2O \rightarrow Na_2ZnO_2$$

Here $$Na_2O$$ is the oxide of the very strong base sodium hydroxide. In simple acid-base language: a metal oxide such as $$Na_2O$$ is a basic oxide. When $$ZnO$$ reacts with this basic oxide, it must be supplying the counterpart—namely, an acidic character—so that a salt, $$Na_2ZnO_2$$ (sodium zincate), can form. Thus in this reaction $$ZnO$$ behaves as an acidic oxide.

Now we examine reaction (ii)

$$ZnO + CO_2 \rightarrow ZnCO_3$$

Carbon dioxide, $$CO_2$$, although a gas, is chemically an acidic oxide because it forms carbonic acid in water. For the two oxides to combine and give the salt $$ZnCO_3$$ (zinc carbonate), the oxide from zinc must now play the opposite role; it has to neutralise the acidic oxide $$CO_2$$. Therefore in this reaction $$ZnO$$ is acting as a basic oxide.

Putting the two observations together:

(i) $$ZnO$$ acts as an acid, and
(ii) $$ZnO$$ acts as a base.

This sequence corresponds to “acid and base,” which matches Option C.

Hence, the correct answer is Option C.