You should know that,

(1)$$\,\,\,$$ In a period, from left to right the acidic strength increases that mean basic nature decreases.

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

Na, Mg, Al and K are present in periodic table like this : -

	Group 1	Group 2		Group 13
Period 3	Na	Mg	.....	Al
Period 4	K

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

And Na and K are from same group so basic nature of K > Na.

So the correct order of oxides

Al₂O₃ < MgO < Na₂O < K₂O