B. prevent European powers from dividing up China.
The United States Open Door Policy was a set of diplomatic principles established in the late 19th and early 20th centuries to protect American commercial interests in China and to promote equal access by all countries to trade and investment opportunities in China. The policy was based on the principle of territorial and administrative integrity of China and aimed to prevent the partition of China by European powers, especially during the period of the Boxer Rebellion in 1900. The policy was also intended to protect American interests in China, which included maintaining access to Chinese markets and resources, ensuring the safety of American citizens and property in China, and promoting American influence in the region.