The "Sweet hereafter" is the idiomatic way to refer to heave, so that explains why the correct answer is B "A beautiful place the dead go, like heaven".
In reference to the novel's title, we can see how this is related owing to the fact that the narrator is telling his story after an accident in which he almost die. We can think that his "sweet hereafter" is the second chance the world give him to not die and keep on living.
