well the pic is opening too small on my machine but if you look carefully in D the shaded area is not aligned completely with the vertical rectangle ...while in your folded pattern explanation it should be. so C is the correct answer ...i will look at it on CPD and will confirm.
It's an error and can be both I think. I know there is an error in the explanation. If you look, there is one shaded black end in the explanation that is all spread out (by end I mean the little squares) but in the question all of the little ends are white.