The correct answer is:
to prevent European nations and Japan from monopolizing Chinese trade.
The Open Door Policy was formulated in the late 19th and early 20th centuries to ensure that all nations would have equal opportunities to trade with China, thus preventing any single power from dominating the Chinese market. This policy was a response to the increasing influence of European powers and Japan in China, which contributed to tensions and ultimately to events like the Boxer Rebellion, as various groups within China reacted against foreign influence and control.