Answer:
Reuniting the Union
Step-by-step explanation:
First, we have to know why Lincoln fought the Civil War: to prevent the complete secession of the Confederacy and to completely unite the Union.
For the first choice, Lincoln said "Now we are engaged in a great civil war" meaning the civil war is still happening. Along with his reason for war, this would make the first choice a plausible answer.
For the second and third choices, he only talked about the dead men who fought at the place of his speech; he never really talked about the cemetery.
For the fourth choice, this speech, the Gettysburg Address, takes place a few months after the Battle of Gettysburg. General Lee was already defeated before this speech.