Industrialization, urbanization (first time in U.S. history that more Americans lived in cities than on farms), jazz (+Harlem Renaissance), Prohibition, flappers/women earning the right to vote/huge increase of women in the workforce, LEISURE TIME (sports, film, etc), cult of the individual hero (i.e. Babe Ruth, Charles Lindbergh, Jim Thorpe). There's a ton you could talk about. But the end of World War One and the urbanization that resulted from it (also caused by the "American Dream"), along with industrialization are probably the most important, and seem to have set the scene for everything else.
So the question is to find the best answer that could support and complete the question and the answer to your question is letter D. Ending their alliances system. I hope you are satisfied with my answer and feel free to ask for more if you have clarification and questions
