Which topic is often used as a theme for studying 20th-century world history

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 is often used as a theme for studying 20th-century world history is "decolonization and nationalism."

This was a time when European colonies started to demand their independence in order to establish their own form of government. One of the best examples could be the Revolutionary War in which the Continental Army defeated the British troops to became independent. Another example is the French Revolution. In Latin America, the best example is the Independence of Mexico from the Spanish crown.

those who allied with france gained land and power.