Final answer:
U.S. Secretary of State John Hay was responsible for explaining the Open Door policy to European countries regarding China.
Step-by-step explanation:
The answer to the question is d) Open Door Policy. U.S. Secretary of State John Hay was responsible for explaining to the European countries the United States' desire to maintain an Open Door policy in China.
This policy aimed to ensure that all countries would have equal trading rights and privileges in China, without any exclusive control or domination by any one nation.
By advocating for the Open Door Policy, the United States sought to secure access to the Chinese markets and prevent other countries from establishing exclusive spheres of influence in China.