Which sentence in this excerpt from Abraham Lincoln's Second Inaugural Address shows that he believed the Union was forced into war to ensure that the country remained united?

Hello. You forgot to put the passage to which this question refers. Here is the excerpt:

On the occasion corresponding to this four years ago, all thoughts were anxiously directed to an impending civil-war. All dreaded it—all sought to avert it. While the inaugural address was being delivered from this place, devoted altogether to saving the Union without war, insurgent agents were in the city seeking to destroy it without war — seeking to dissolve the Union, and divide effects, by negotiation. Both parties deprecated war; but one of them would make war rather than let the nation survive; and the other would accept war rather than let it perish. And the war came.

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. All knew that this interest was, somehow, the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union, even by war; while the government claimed no right to do more than to restrict the territorial enlargement of it. Neither party expected for the war, the magnitude, or the duration, which it has already attained. Neither anticipated that the cause of the conflict might cease with, or even before, the conflict itself should cease. Each looked for an easier triumph, and a result less fundamental and astounding. Both read the same Bible, and pray to the same God; and each invokes His aid against the other. It may seem strange that any men should dare to ask a just God's assistance in wringing their bread from the sweat of other men's faces; but let us judge not that we be not judged. The prayers of both could not be answered; that of neither has been answered fully.

"Both parties deprecated war; but one of them would make war rather than let the nation survive; and the other would accept war rather than let it perish."

Explanation:

With the phrase shown above, Lincoln shows how the entire nation feared war, but was divided with the concepts of sovereignty and power, making part of the country (the confederates) fear war, but were willing to start it, even that this would bring about the end of the nation. Because of this thought, the union (which did not want to wage war under any circumstances) was forced to join the fight and fight those who thought war was a solution. Only with the participation of the union would the country remain united and that was a sacrifice that the union had to make.

the answer is c. to explore the louisiana purchase