The base of the Pyramid is a rectangle and you know that the plane that intersects the pryamid is parallel to its base. So the cross section will be also a rectangle.
If you take z=0 (floor or the pyramid base)
Put the plane in z=0 the cross section will be the rectangular base. it is also parellel to the base
Now start to move the plane on the Z axis the cros section will continue to be a rectangle is just is section that changes.
