Which Factor Contributed to the Absence of Major War on the Continent

Which factor contributed to the absence of major war on the continent under the Concert of Europe?

A) Italy disintegrated into multiple fragments,leading to a balance of power between the other European states.
B) Colonialism allowed rivalries to be played out in distant places.
C) The United States rose significantly in power,balancing against the countries of Europe.
D) The masses overthrew their rulers and brought about democracy on the continent.