AIEEE 2011 Solved Paper

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 :-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