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

a. the american civil war was an extended battle engagement fought between the confederacy and the union in the nineteenth century.
b. the american civil war was an extended battle engagement fought between the confederacy and the union in the nineteenth century.
c. the american civil war was an extended battle engagement fought between the confederacy and the union in the nineteenth century.
d. the american civil war was an extended battle engagement fought between the confederacy and the union in the nineteenth century.