The correct answer is D) Decolonization and nationalism.
The other options of the question were A) Diminishing civil rights protections. B) Political and economic isolation. C) Peaceful resolution of conflicts.
The topic that is often used as the theme for studying world history since World War 1 is "Decolonization and nationalism."
During this time in world history, powerful countries that used to colonize other territories had to leave them or recognize the independence movements in places such as Africa and Asia. A wave of nationalism started to hit these nations that established their own form of government. The process was not an easy one because after many years of European ruling when they got their independence what resulted was much rebellion, conflict or civil wars.