How did the war between the states redefine america?

The Civil War ended the idea that a state could break away (secede). By defeating the Confederacy, the Union had preserved the America as "One Nation". There would be no more discussion of secession (leaving). It also, once again, established the national government as the head. Loyalty to your state would come second to your loyalty to the nation. It also helped the South to diversify their economy, since most of it was almost completely destroyed. 

Ithink the answer is d but don’t answer it. i need points.