The correct answer would be option B: "The Civil War", this is because it is the only option that affected primarily the southern part of USA and forever changed their society and way of life. The Reconstruction era was a time after the civil war, in which both parts (north and south) of the country tried to move on with their lives. The Economic Depression was an economic crisis that started in 1929, affecting the whole country. Therefore options A, C and D are discarded as correct answers.