Final answer:
The Harlem Renaissance, with its significant cultural contributions through arts, literature, and music, made the largest impact on American culture after World War I, influencing not just African American culture but the broader American cultural landscape as well.
Step-by-step explanation:
In assessing the largest contribution to the development of American culture after World War I, the Harlem Renaissance stands out. This cultural movement embodied a significant period of cultural formation for African-Americans, marking a profound contribution to American arts, literature, and music. Artists and intellectuals such as Langston Hughes and W.E.B. Du Bois used this time to express the richness and uniqueness of black culture, deeply influencing the broader American cultural landscape.
While the Prohibition Era led to a rise in organized crime and illegal bootlegging, and the Roaring Twenties brought about a more permissive morality, the Harlem Renaissance provided a surge in creativity and expression that redefined the country's artistic and social outlook. Moreover, the Red Scare added to the era's turmoil, but it was the cultural rebirth in Harlem that echoed tolerance and understanding through the arts, setting a precedent for future civil rights movements.