The Roosevelt administration strove to avoid Woodrow Wilson's mistakes in selling the League of Nations to the Senate. It sought support from both parties, and in September 1943 the Republican Party endorsed U.S. participation in a postwar international organization, after which both houses of Congress overwhelmingly endorsed participation. Roosevelt also sought to convince the public that an international organization was the best means to prevent future wars. The Senate approved the UN Charter on July 28, 1945, by a vote of 89 to 2. The United Nations came into existence on October 24, 1945, after 29 nations had ratified the Charter. Background information: This passage discusses how the United States ratified the charter of the United Nations and became a founding member. What convinced the United States to join the United Nations? The achievements of the League of Nations convinced people the UN could succeed. The House of Representatives overrode the Senate’s objections to the organization. Both political parties, and the public, agreed that it would help prevent future world wars. President Wilson worked with President Roosevelt to ensure US membership.

Option C. What convinced the United States to join the United Nations is that both political parties, and the public, agreed that it would help prevent future world wars.

Explanation:

As the text shows, The Senate of the United States, after a congressional debate, officially approved the UN Charter on July 28, 1945, and with its approval a historic moment for the United States and for international cooperation was settled. The main reason that convinced the United States' Congress as well as the public opinion, that the country should join the United Nations was that a majority believed that the organization would help prevent future wars, and that it was important to learn from the mistakes made previously, with the League of Nations, when after the Senate debated whether or not the country should participate in the organization, it was voted that it shouldn't and then a major agent such as the United States was not a part of an international organization that had the goal to maintain world peace.