Final answer:
The 13th Amendment is the constitutional amendment that ended slavery in the United States, making it illegal except as punishment for a crime.
Step-by-step explanation:
The correct answer to which constitutional amendment ended slavery in the United States is a) 13th Amendment - Ended slavery. Ratified after the Civil War, the 13th Amendment officially abolished slavery and involuntary servitude, except as a punishment for a crime. It was a landmark in American history, ensuring that the practices associated with slavery would not continue in the United States. The 14th Amendment, on the other hand, guaranteed equal protection under the law and defined citizenship, while the 15th Amendment focused on voting rights, prohibiting the federal and state governments from denying a citizen the right to vote based on race, color, or previous condition of servitude. Lastly, the 19th Amendment, ratified much later, granted women's suffrage, thereby extending voting rights to women.