The correct answer is C) Slavery was an established institution until the Twelfth Amendment was ratified on December 18, 1865.
<em>The statement that is not true is "Slavery was an established institution until the Twelfth Amendment was ratified on December 18, 1865."</em>
This statement is not true.
The amendment that abolished slavery was the 13th amendment, not the 12th.
The 13th amendment of the United States Constitution abolished slavery. The Senate approved the amendment on December 1864. The House of Representatives approved it one month later, on January 31, 1865.
The exact text of the 13th amendment says: "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subjected to its jurisdiction."