The correct answer is D.
The Washington Disarmament Conference was a military conference organized by US President Harding in 1921. It was called outside the mechanisms of the League of Nations, and nine nations were invited to participate: US, China, Japan, Britain, France, Italy, Belgium, Portugal and Netherlands. Russia and Germany were not invited.
For the first time in history, the purpouse of the meeting was to establish a limitation on weapons' production.