D should be correct. Hope I helped, tell me if I am incorrect :3
The dialogue shows that Furtunato thinks Montrsor is playing a joke on him.
I would say the answer is "harbor". This word is usually used as a noun; but in this case, it is a verb that means to provide shelter to someone or something, to keep someone safe, but also to have something in mind and think about it. This word is purposefully chosen and fits into Whitman's well-intended, philanthropic and inclusive worldview.