Which of the following sentences demonstrates proper capitalization? [L.11-12.2]

AThe American Civil War was an extended battle engagement fought between the confederacy and the union in the nineteenth century.

BThe American Civil War was an extended battle engagement fought between the Confederacy and the Union in the Nineteenth Century.

CThe American civil war was an extended battle engagement fought between the confederacy and the union in the nineteenth century.

DThe American Civil War was an extended battle engagement fought between the Confederacy and the Union in the nineteenth century.