I’m pretty sure a question that you can’t answer
Well, since you didn't include the excerpts, I cannot possibly know which ones you are referring to, but overall, Dickinson used iambic tetrameters and iambic trimeters in this poem.
In my opinion, the correct answer is B. His mystery feeds their imagination. The children have never even seen Boo, so they like to imagine and envision different spooky details about him. Jem fantasizes that he catches and eats squirrels and cats, which is why his hands are always bloody; he drools, his teeth are rotten, he has a dreadful scar on his face, etc.