The Death of Ivan Ilyich is a book written by Russian author Leo Tolstoy. The book is about how Ivan Ilyich tries to climb the social scale. In this case, the excerpt is almost at the very end of the book. In this case, the deep black sack symbolizes death, seen when the narrator says "they were pushed further and further in they could not be pushed to the bottom;" since death seems endless, it is the correct answer.
He helps her with her fear of monsters by going along with it. He calls it the Imaginary Demon. By making her fear seem like a "real" thing, it adds humor to it and makes it less frightening.