Basic nature of oxides across a period

Metallic oxides are generally basic, while amphoteric oxides show weaker basic character.

• Alkali metal oxides like $Na_2O$ and $K_2O$ are strongly basic.
• Alkaline earth metal oxide like $MgO$ is basic, but less basic than alkali metal oxides.
• $Al_2O_3$ is amphoteric, so it is the least basic among these.

Compare the given oxides

• $Al_2O_3$ : amphoteric, hence least basic
• $MgO$ : basic
• $Na_2O$ : strongly basic
• $K_2O$ : more basic than $Na_2O$

Why is $K_2O$ more basic than $Na_2O$?

Down Group 1, metallic character increases, so the oxides become more ionic and more strongly basic.

Hence: $K_2O > Na_2O$

Overall increasing order of basic nature

Therefore, the correct increasing order is $Al_2O_3 < MgO < Na_2O < K_2O$

Match with options

This corresponds to Option B.

Verification with stored answer

Stored correct answer: B

Derived answer: B

So, the derived answer agrees with the stored answer.