Which of the following constitutional amendments formally ended slavery at the end of the Civil War?

The Twelfth Amendment, ratified in 1804, primarily addresses the procedures for electing the President and Vice President of the United States. It does not pertain to slavery or civil rights, making it irrelevant to the question regarding the abolition of slavery.

Ratified in 1868, the Fourteenth Amendment granted citizenship to all persons born or naturalized in the United States and provided equal protection under the laws. While it addressed civil rights and protections for former slaves, it did not directly end the practice of slavery.

The Fifteenth Amendment, ratified in 1870, prohibits the federal and state governments from denying a citizen the right to vote based on "race, color, or previous condition of servitude." Although it aimed to protect voting rights for African American men, it does not relate to the abolition of slavery itself.

The Sixteenth Amendment, ratified in 1913, allows the federal government to impose an income tax. This amendment has no connection to slavery or its abolition, thus making it unrelated to the question about ending slavery.