The correct option is C) Slavery was an established institution until the Twelfth Amendment was ratified on December 18, 1865.
This option is actually wrong since the twelfth amendment actually provides the procedure to elect the President and Vice President of the United States.
Other options in the question are true. Dr. Martin Luther King, Jr. DID give the famous "I Have a Dream" speech in 1963 which has now become ingrained in US culture.
Also, In 1870 the Fifteenth Amendment gave all male citizens the right to vote. Before this amendment, not everyone really had the right to vote and it would be another 90 years until women got the right to vote.