The last major country to join the Allied Powers during World War II was China, which strengthened its alliance with the Allies after the United States entered the war. Although China had been engaged in a conflict against Japan since 1937, its formal alignment with the Allies occurred in December 1941. The other countries listed had already been involved in the alliance earlier in the war. ;
The last significant country to join the Allied Powers in terms of formal entry during World War II was the United States. Although China had been engaged in conflict against Japan since 1937, it was not considered a 'joiner' until the U.S. entered in December 1941. Therefore, the chosen answer is C. United States.
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