The issue addressed in the post-Civil War amendments primarily pertains to citizenship. Specifically, the 14th Amendment, ratified in 1868, granted citizenship to all persons born or naturalized in the United States, including former enslaved individuals. It also established equal protection under the law. The 13th Amendment abolished slavery, while the 15th Amendment, ratified in 1870, aimed to secure voting rights for African American men.
Thus, the correct answer is citizenship.