$(2)$ In a group, from top to bottom the basic nature increases that means acidic nature decreases.

$Na, Mg$ and $Al$ are present in same period so the order of basic nature among and $Al$ is $Na> Mg > Al.$

And $\mathrm{Na}$ and $\mathrm{K}$ are from same group so basic nature of $\mathrm{K}>\mathrm{Na}$.

So the correct order of oxides

$\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{MgO}<\mathrm{Na}_{2} \mathrm{O}<\mathrm{K}_{2} \mathrm{O}$