I would say B) SIGHT, but I'm not fully sure, because this is a confusing question, sort of. In the beginning, the (admittedly insane) narrator rambles on about how she is nervous, and how cunning she believes herself to be, making sure to walk into the old man's room every night, observing the pale blue vulture eye beyond retrospect, beyond the line. It sort of gives me the idea that she relies on sight in the beginning of the story, but I very well may be wrong.