The Allies created the United Nations after World War II to

prosecute and punish war criminals.

subdue and take over aggressive nations.

establish free international elections and rules in war.

maintain international peace and security.

Maintain international peace and security

The correct answer is maintain international peace and security.

After world war 2 the countries of the world realized that they had to prevent another such war from ever happening again. They tried to do it even after world war 1 with the league of nations but that project failed as it didn't have the support of various armies from the world such as the US army. The United Nations however got even Americans on board and still exists today.