Which topic is often used as a theme for studying world history since world war 1

The options to this question are:

A. Diminishing civil rights protections B. Political and economic isolation
C. Peaceful resolution of conflicts
D. Decolonization and nationalism
The answer is Decolonization and nationalism, which is D.
The other part of the century was characterized by countries formally held by colonial powers agitating for self determination.From Africa,India to Asia. This era, globally is better referred to as the era of nationalism, beginning from the 1920's and culminating in the 1990's with the end of apartheid in south Africa.

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The re-election of the president abraham lincoln.