C. Her goodness made her unique compared to most slave owners.
Explanation:
They're describing the harsh treatment that Douglass received from the mistress. Basically dehumanizing is treating another human inhumanly. Treating someone in an inhumane way also means to be cruel. also, you can use context clues from the paragraph "when I first met her at the door,--a woman of the kindest heart and finest feelings." This is past tense.
