The line that signifies the narrator's turn from unfavorable to favorable descriptions of the "dark lady" is D. "And yet, by heaven, I think my love as rare."
The other options describe her as nothing special to the author - he is saying that her voice is horrible, that her eyes are ugly, and that she walks really loudly. But the last line states that he loves her no matter what, despite her flaws of which there are many.
