Correct answer: A) He believed that the expansion of slavery endangered the Union.
History/context:
In a speech Lincoln gave during the 1858 campaign when Lincoln ran for the US Senate, challenging Illinois' incumbent Senator Stephen Douglas, Lincoln had said of slavery, "Although volume upon volume is written to prove slavery a very good thing, we never hear of the man who wishes to take the good of it, by being a slave himself." Lincoln was morally opposed to slavery, but he also recognized that slavery was permitted by the existing law of the land, the US Constitution. So Lincoln's initial position on slavery was to stop the spread of it.
Lincoln did not manage to unseat Stephen Douglas from his seat in the Senate in 1858. However, Lincoln did succeed in winning election as President in 1860,
In the South, many feared an activist anti-slavery agenda from Lincoln and the Republicans. According to the History Channel, "The election of Abraham Lincoln was labeled an act of war by some Southern politicians, who predicted armies would come to seize slaves and force white women to marry black men." So as soon as Lincoln was elected, planning for secession began to occur in states across the South. By February, 1861, seven states had succeeded. Representatives of those seven states (South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas) met in Montgomery, Alabama, and formed the Confederate States of America. Four more states joined the Confederacy later in 1861: Virginia, Arkansas, North Carolina, and Tennessee.
The progress of the Civil War made Lincoln increasingly strong in his stance against slavery. The war initially was about preserving the Union, but later, with Lincoln's Emancipation Proclamation (1863), was declared also to be about ending slavery.