The correct answer to this open question is the following.
The question does not provide options or another specific reference. However, we can comment on the following.
Abraham Lincoln considered that the problems that couldn't be solved individually were the issue of slavery in the southern states, the unresolved conflicts that the entrance of Nebraska, Missouri, and Kansas to the Union, and the idea of some states to secede from the Union.
Those are my three examples.
Abraham Lincoln was very aware of those above-mentioned issues. He even expressed in one of his speeches as a Republican candidate that "A house divided against itself, cannot stand."