The United States advocated the Open Door policy because

The primary reason the US advocated the Open Door Policy was to protect and broaden US trade with China. This policy was put into effect in the early 20th century after the Boxer Rebellion in China. It hastened the end of the last dynasty in Chinese history.

Explanation:

Open Door Policy of Secretary John Hay aims to keep China a free trade country with equal trade opportunities between countries. At that time, the European country has colonized nearby Hongkong. Thus, the answer is letter C. Open Door policy had kept European Power to limit foreign trade with China.

b

being given a fair trial before a jury