Which of the following is an amphoteric oxide?

As stated in the NCERT chapter (Section 3.2.1), most metal oxides are basic, but "some metal oxides, such as aluminium oxide, zinc oxide show both acidic as well as basic behaviour. Such metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides."

$$Al_2O_3(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2O(l)\quad(\text{basic behaviour, reacts with acid})$$

$$Al_2O_3(s) + 2NaOH(aq) \rightarrow 2NaAlO_2(aq) + H_2O(l)\quad(\text{acidic behaviour, reacts with base})$$

Since $Al_2O_3$ reacts with both an acid and a base to give salt and water, it is amphoteric — hence option D is correct.

Why the other options are wrong:

• A. $Na_2O$: This is a strongly basic oxide. The book gives $Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)$ — it dissolves in water to form an alkali and reacts only with acids, not bases.
• B. $K_2O$: Also a strongly basic oxide (analogous to $Na_2O$): $K_2O(s) + H_2O(l) \rightarrow 2KOH(aq)$. It does not react with a base.
• C. $CO_2$: Carbon is a non-metal, and the chapter states non-metals "produce acidic oxides" when dissolved in water. $CO_2$ is an acidic oxide (reacts only with bases, not acids).