Final answer:
The Thirteenth Amendment to the United States Constitution completed the process begun by the Emancipation Proclamation, officially abolishing slavery in the entire country in December 1865.
Step-by-step explanation:
The constitutional amendment that completed the process begun by the Emancipation Proclamation was the Thirteenth Amendment. President Abraham Lincoln supported the movement for an amendment to give emancipation the force of law. The Thirteenth Amendment, which abolished slavery in the country, was passed by Congress in early 1865, swiftly gained the necessary support of the states, including in the South, and was officially ratified in December 1865. This monumental step finalized the abolition of slavery throughout the United States.