The oxide that shows magnetic property is:

Among the given oxides, we identify which shows magnetic properties. $$\text{SiO}_2$$ is a covalent network solid and is diamagnetic. $$\text{Na}_2\text{O}$$ and $$\text{MgO}$$ are ionic compounds with all electrons paired, making them diamagnetic.

$$\text{Mn}_3\text{O}_4$$ (hausmannite) is a mixed-valence oxide containing $$\text{Mn}^{2+}$$ and $$\text{Mn}^{3+}$$ ions, both of which possess unpaired d-electrons. It adopts a spinel structure and exhibits ferrimagnetic behaviour, giving it a net magnetic moment.

The correct answer is option (2): $$\text{Mn}_3\text{O}_4$$.