Which oxide is amphoteric?

The correct option is BeO.

1. Definition of Amphoteric Oxides:
An amphoteric oxide is a chemical compound that displays both acidic and basic behaviors. This means it can react with both acids and bases to form salt and water.

2. Periodic Trends in Group 2 Oxides:
The given options contain oxides of elements from Group 2 (the alkaline earth metals): Beryllium ($\mathrm{Be}$), Magnesium ($\mathrm{Mg}$), Calcium ($\mathrm{Ca}$), and Barium ($\mathrm{Ba}$).
As we go down the group in the periodic table, the metallic character of the elements increases, which causes the basic character of their oxides to increase as well:

$\mathrm{BeO}$ is amphoteric.

$\mathrm{MgO}$ is weakly basic.

$\mathrm{CaO}$ and $\mathrm{BaO}$ are strongly basic.

3. Chemical Explanation:
Beryllium has a very small atomic size and a relatively high electronegativity compared to the other Group 2 elements. Because of its high charge density, the bond between Beryllium and Oxygen has significant covalent character, unlike the ionic bonds found in the other alkaline earth metal oxides. This allows beryllium oxide to react with both strong acids and strong bases, demonstrating its amphoteric nature.

Reaction with Acid (e.g., Hydrochloric Acid):

$$\mathrm{BeO}+2\mathrm{HCl}\to \mathrm{Be}{\mathrm{Cl}}_2+H_{2}O$$

Reaction with Base (e.g., Sodium Hydroxide):

$$\mathrm{BeO}+2\mathrm{NaOH}+H_{2}O\to {\mathrm{Na}}_2\left[\mathrm{Be}{\left(\mathrm{OH}\right)}_4\right]$$

Because Beryllium Oxide undergoes both types of chemical reactions, it is classified as amphoteric, whereas the remaining oxides (MgO, CaO, BaO) are exclusively basic.