Final answer:
The United States developed the Open Door policy to promote the expansion of US trade with China.
Step-by-step explanation:
The United States developed the Open Door policy to promote the expansion of US trade with China. This policy aimed to ensure equal trading opportunities for all countries within China and prevent the colonization and division of China by foreign powers. It also aimed to protect American interests in the region by maintaining access to Chinese markets. The Open Door policy was not specifically designed to encourage Chinese migration to the US, aid Chinese Communists or Nationalists, or demonstrate the superiority of democracy.
Learn more about Open Door policy in US-China relations