Which amendment declared slavery illegal in the United States?

The 13th Amendment is the correct answer because it explicitly abolished slavery and involuntary servitude in the United States. Ratified in December 1865, following the Civil War, this amendment marked a significant turning point in American history by ensuring that no person could be legally held as a slave or forced to work against their will, except as punishment for a crime.

The importance of the 13th Amendment lies in its clear and decisive language, which transformed the legal status of millions of African Americans and laid the groundwork for future civil rights advances. The amendment reflects the core values of freedom and equality, which are foundational principles in the United States.

In contrast, the 14th Amendment primarily focuses on citizenship rights and equal protection under the law, while the 15th Amendment addresses voting rights, prohibiting the denial of the right to vote based on race. The 12th Amendment relates to the electoral process in the United States and has no direct connection to slavery or civil rights. Thus, the 13th Amendment is the definitive answer in the context of abolishing slavery.