Question

US Secretary of State John Hay established the policy, which opened China s ports to European trade

Answer 1

Answer;
The open door policy

Explanation;
Us secretary of state John Hay established the Open door policy which opened China's ports to European trade. 
Open Door policy was a statement of principles initiated by the United States in 1899 and 1900 for the protection of equal privileges among countries trading with China and support of Chinese territorial and administrative integrity. John Hay, the secretary of state proposed this policy under president McKinley towards China for all countries.

Answer 2

US Secretary of State John Hay established the "Open Door" policy, which opened China's ports to European trade, since the US wanted to benefit from the plethora of trading opportunities that dealing with China offered.