Of the choices that you have given here, the last choice is without question the correct answer. The war, particularly at the beginning, was always about preserving the Union.
We often see the war as a war that was meant to end slavery. But this was not the main goal. Lincoln himself famously told Horace Greeley that if he could save the Union by not freeing any slaves, he would do it. He would also do it by freeing some of the slaves or all of the slaves. The point was that Lincoln was focused mainly on saving the Union. The freeing of the slaves was a result of saving the Union, not the main goal of the war.