Answer: To reference the Civil War document that ended slavery
Step-by-step explanation:
The Lincoln Memorial evidently was erected to honor the memory of Abraham Lincoln who through victory in the American Civil War over the Confederacy, was able to free all slaves in the United States in what we know as The Emancipation Proclamation and the subsequent 13th Amendment which was passed shortly after the Civil War ended.
In giving a speech next to a memorial for President who made this possible, Martin Luther King was attempting to reference the document that President Lincoln signed that ended slavery.