From the figure attached with this answer have octagon ABCDEFGH.
We can make a cut to create a plane section in the following shapes:
1. Rectangle
Here ADEH form a rectangle. To make rectangle we take ADEH
2. Octagon
For getting Octagon just cut ABCDEFGH form octagon
3. Triangle
To get triangle cut opposite diagonals. From the figure we can see AOB,BOC,COD,DOE,EOF,FOG,GOH and HOA are triangles.
4. Trapezoid
To get trapezoid ABCD form trapezoid and EFGH form another trapezoid
