Option D is the right answer, that they opposed it because the isolationist sentiment was growing, and many people feared American involvement in another war.
At the Paris Peace Conference of 1919, the first International Peacekeeping Organization was established by President Woodrow Wilson with an idea to solve International disputes. Although president Wilson enthusiastically promoted this organization yet the United States did not join the league officially due to the Congress's Isolationists Opposition. President Wilson met vigorous opposition from this group, particularly from Republican senators, who protested to the Article X of the league's agreement. Isolationist opposed the United States further involvement in any international conflicts and saw Article X as the direct negligence of US Sovereignty.