Oxide of the metal cation which is not amphoteric

The answer is 3. The oxide of Mn²⁺ is not amphoteric.

Here is a table of the amphoteric oxides:

Metal Cation	Oxide
Al3+	Al2O3
Cr3+	Cr2O3
Zn2+	ZnO

The oxides of Al³⁺,Cr³⁺ and Zn²⁺ are all amphoteric. However, the oxide of Mn²⁺ is not amphoteric. This is because Mn²⁺ has a relatively low charge, which means that it does not have a strong polarizing effect on the oxygen atoms in the oxide ion. As a result, the oxide of Mn²⁺ is not a strong acid or base, and it does not dissolve in either acidic or basic solutions.