Final answer:
The Open Door Policy resulted from the attempts of European powers to control trade in China, leading to U.S. intervention to ensure equal trading rights.
Step-by-step explanation:
The Open Door Policy resulted primarily from A. the attempts of European nations to control trade in China. As foreign powers divided China into spheres of influence, the United States, fearing exclusion from Chinese markets, proposed the Open Door Policy to ensure all nations could trade on equal terms in China. Following the Boxer Rebellion, European powers accepted the policy to avoid further conflict and maintain their trade interests without claiming more Chinese territory. This policy aimed to respect China's sovereignty while allowing free trade.