I'm assuming you have the octagonal base flat on the ground/floor. If so, then the horizontal cut through the center would basically cut the prism into two smaller equal pieces. Each piece will have the same octagonal base, but now the height is half as much. The cross section is the same as the base.
One way to picture it is to imagine you had a single room house with a very tall ceiling. Now place a floor between the top and bottom, midway through, so that you divide this house into too stories (each story being the same height). This new floor is a cross section, which is identical to the original floor of the house. Instead of a rectangular floor, picture the floor shape being an octagon.
Answer: Octagon