Final answer:
The main reason the United States developed the Open Door policy was to promote trade with China.
Step-by-step explanation:
The main reason the United States developed the Open Door policy was to promote trade with China. The Open Door policy was a response to the division of China by European powers into spheres of influence, which threatened to exclude the United States from accessing Chinese markets and ports. The United States wanted to ensure that it had equal trading rights in China and that its economic interests were protected.