The correct answer is C) The U.S., as the world's richest nation, was vital to any international agreement.
The United States was at the forefront in negotiating many of the international treaties of the 1920s because the U.S., as the world's richest nation, was vital to any international agreement.
During the "roaring 1920s," the United States lived a period of economic expansion that placed them as the world's richest nation at that time. Powerful countries in Europe such as England and France were just ending a difficult period for Europe after World War 1, and they received loans from the United States that helped them to support the war effort. So the presence of the United States was obligated in any international treaty to back the agreements.
The other options of the question were A) the US was regarded as a fair and neutral nation. B) Americans believed they could outlaw war. D) Presidents Harding and Coolidge had a plan to bring the US into the League of Nations.