D. The Klan encouraged terrorism of white citizens to incite racial violence against African Americans.
While the Ku Klux Klan did engage in acts of terrorism and violence, their primary focus was not on terrorizing white citizens, but rather on promoting white supremacy and inciting violence against African Americans and other minority groups. This makes option D not a cause of the Klan's resurgence in the 1920s compared to the other options listed.