The correct answer is D. United States.
After World War II, the United States invested heavily in the rebuilding of Europe and Japan through the Marshall Plan and other initiatives. The Marshall Plan, also known as the European Recovery Program, was a program implemented by the United States to provide financial aid to European countries that had been devastated by the war. The plan was designed to help these countries rebuild their economies and prevent the spread of communism in Europe.
In addition to the Marshall Plan, the United States also provided economic and military aid to Japan as part of the rebuilding process. This aid helped Japan rebuild its economy and become a major economic power in the postwar period.
Overall, the United States played a key role in the rebuilding of Europe and Japan following World War II, making it the correct answer to the question.