Final answer:
The United States became the sole world superpower after the end of the Cold War in 1991 when the Soviet Union collapsed. So the correct option is D.
Step-by-step explanation:
The United States became the sole world superpower following d. the end of the Cold War. This marked a significant shift in the global power structure, as the bipolar world dominated by both the USA and the Soviet Union transformed into one with a singular superpower. The Cold War, stretching from the end of World War II in 1945 until the collapse of the USSR in 1991, was an era of significant tension and competition between the two nations, where they vied for military, economic, social, technological, and ideological supremacy. With the dissolution of the Soviet Union, the USA emerged as the world's only superpower, taking on a hegemonic role in global politics.