The right answer is "d. The United Nations had the support of most Americans".
The explanation for this is that after World War II there was a sense of loss everywhere and the population wanted something that could prevent further conflicts like the WW2. Thus, in 1945, the UN was created to promote cooperation between nations, in an international way, in order to guarantee peace and the organizations taking place in a positive way in the world.
Because of the UN objectives, Americans have been very positive to the UN, as well as other citizens around the world.