So if you look at your result from part 3, there is a figure in the lecture notes that plots that exact dispersion relation:
You know that the atoms are monovalent and that the max number of electrons per energy band is 2, so the energy bands are half filled. The answer plot is a top down view (which I think is kind of hard to visualize), but essentially if you look at the ‘3D’ graph, and trace out the halfway points on the energy curve you get:
which is the rectangle they’re talking about. If you decrease the number of electrons per band the amount the energy band is filled decreases which traces out an elliptical shape.