What was king referring to when he said, “the long night of their captivity” in paragraph one

He turned into speakme about the wish that was given to slaves while the emancipation proclamation changed into exceeded and for a long time, it did nothing. It came as a joyous daybreak to quit the lengthy night of their captivity." Metaphor: The Emancipation Proclamation is in comparison to a "fantastic beacon light of hope." The injustice of slavery is in comparison to searing flames. The Emancipation Proclamation is compared to a joyous daybreak after a long night.

The answer has to be D