Step 1: Understanding the Open Door Policy.
- The "Open Door Policy" was a term used primarily by the United States in the late 19th and early 20th centuries, aimed at ensuring equal trading rights for all nations in China, and maintaining China's territorial integrity. This policy was articulated by Secretary of State John Hay in 1899.
Step 2: Analysis of Options.
- (A) China: China did not adopt an "Open Door" policy. It was subject to the policy imposed by the USA and other countries.
- (B) EU: The European Union did not adopt the "Open Door" policy.
- (C) Japan: Japan did not adopt the "Open Door" policy either.
- (D) USA: Correct. The "Open Door" policy was adopted by the United States to ensure open access to trade in China.
Step 3: Conclusion.
Thus, the correct answer is (D) USA.