The main reason the United States implemented the Open Door policy in China was to guarantee access to markets encourage religious freedom expand democratic reforms promote immigration

The main reason the United States implemented the Open Door policy in China was to **guarantee access to markets**. The Open Door policy, proposed by the United States in the late 19th and early 20th centuries, aimed to ensure that all countries had equal access to trade and investment opportunities in China, particularly in the wake of growing European imperialist ambitions in the region. This policy sought to protect American commercial interests and prevent any single country from dominating China's economy.