The appositive is (B) Mary Poppins, because there are "invisible" understood commas in that sentence if you know what I mean! ;)
Original: P.L. Travers's book Mary Poppins was recommended to me by the librarian.
Choice A: P.L. Travers's book Mary Poppins was recommended to me by the, librarian.
Choice B: P.L. Travers's book, Mary Poppins, was recommended to me by the librarian.
Choice C: P.L. Travers's book Mary Poppins was recommended to me by, the librarian.
Choice D: P.L. Travers's book, Mary Poppins was recommended to me by the librarian.
B is proper punctuation. I hope this helps!
