Remark
Draw a triangle on a piece of paper. Make sure the base is perfectly horizontal (let it be drawn on a line on your piece of paper if the paper is lined). Now a little ways up, draw another line parallel to the base.
The two triangles, the little one and the big one can be shown to be similar.
If you have 4 triangles that are the same as the one you originally drew and you glue them together, you get 4 lines that are parallel to the base. If the base is a rectangle or a square, then what you cut will be a similar rectangle or square on its cross section.
Put another way the cross section will be the base of the small pyramid that has been sliced away from the big pyramid.
Answer A rectangle
