The correct answer is D. A soon to come separation
Step-by-step explanation:
The context of a word refers to the background information and the phrases and words that surround a specific word or expression in a text and tells the reader the sense or meaning of the word. In this case, the text presented in the question describes some of the elements that caused the American Civil War which was a historical conflict from 1861 to 1865 between the Northern and the Southern States in the U.S. Because of this, the narrator explains the role of Abraham Lincoln, the way the Union of the states began being in danger and the dissatisfaction of the Southern states as they wanted to protect slavery, situation that led to resolutions to protect slavery but in 1860 with the victory of Lincoln caused the "subsequent secession of the more radical southern states".
Considering the term "subsequent secession" is related to the background information about the conflict between the South and the North that led to the Civil War and it is also connected to the words "split", "drifting apart" and "a line of real division" the background information and other words that belong to the context suggests the "subsequent secession" refers to the separation of the Southern states from the union that occurred after the victory of Lincoln. Thus, in this context, the phrase "subsequent secession" indicates a soon to some separation.