The answer is A. Two European empires were dismantled.
World War I led to the dismantling of several empires in Europe, most notably the Austro-Hungarian Empire and the Ottoman Empire. The Treaty of Versailles and other treaties that followed the war resulted in significant territorial losses and the creation of new nations, fundamentally altering the political landscape of Europe.
This option is incorrect. In fact, American influence in Europe increased after World War I, particularly through the United States' involvement in the peace process and the League of Nations, although the U.S. Senate ultimately did not ratify the Treaty of Versailles.
This option is incorrect. While World War I was a devastating conflict, it did not result in a series of genocides in Western Europe. The Armenian Genocide occurred during this period, but it was carried out by the Ottoman Empire, not in Western Europe.
This option is incorrect for the period immediately following World War I. While the Russian Revolution led to the establishment of a communist government in Russia, communism did not take hold in most of Eastern Europe until after World War II, when the Soviet Union expanded its influence over the region.