D makes you think that there was a second civil war. MLK was not angry with Lincoln: History had taught him that the Emancipation Proclamation was the first step. D is not true.
Of all your choices, this one could be eliminated the most quickly. The Civil Rights Movement showed that African Americans were anything but wholly satisfied. C is incorrect.
The best answer is B. Sometime when you have a moment read "I have a dream." speech delivered by King.
It wasn't slavery exactly. It was segregation mostly. A is not a good answer: it might work if B was not better.