Final answer:
The Open Door policy, proposed by U.S. Secretary of State John Hay in 1899, ensured equal trade access to all nations in China and preserved China's territorial integrity.
Step-by-step explanation:
The proposal that ensured equal access to the Chinese market for all nations and preserved the unity of the Chinese Empire was known as the Open Door policy. In 1899, U.S. Secretary of State John Hay introduced the Open Door notes, seeking fair competition among international powers within China.
This policy maintained that all nations should have the opportunity to trade with China on equal terms, without any one power gaining control over large swathes of territory through colonization or otherwise forming exclusive spheres of influence. As a result, China agreed to trade with all countries on the same terms and none of the industrialized powers could gain an advantage over the others. In exchange for this openness in trade, the foreign powers agreed not to annex any portion of Chinese territory, thus preserving its sovereignty.