The correct answer is D) thinks that discussing his wife’s behavior is beneath him.
Exaplanation:
In the line 34, The Duke says that he does not “stoop to blame” the Duchess for her actions. It means that he does not bow down himself to tell her how he disapproves her behaviour. The meaning of stoop is to bow down to the same level of other person, so the duke will never lower himself to the Duchess’s level to blame her.