Answer:
H. Slavery.
Step-by-step explanation:
The Second Inaugural Address of President Abraham Lincoln was given in 1865 after the Civil War had occurred. This war would be one of the most important wars fought between the Union and the Confederates that will determine and shape the future of the United States.
In the given excerpt, President Lincoln mentioned "this interest" as being the "cause of the war." Now, we know that the war he was referring to was the Civil War, and the main issue about that war was that of slavery. Moreover, the lines before the given excerpt go like this,
"One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the Southern part of it. These slaves constituted a peculiar and powerful interest."
So, we can safely infer that the "interest" that he said refers to the issue of slavery.
Thus, the correct answer is option H.