The correct answer is A: cultured manners and evil nature. The entire shock of Browning's poem comes from the contrast between the duke's refined manners and the fact that he could order his wife's murder so blithely and on such minor grounds. Not only that: he relates murder as if it were a minor housekeeping issue. We normally associate murder with brutality and ignorance (and, one might add, with the lower classes), and the duke challenges those assumptions: he is a refined, cultured murederer who doesn't think twice about offing his wife.