A trapezoidal prism is shown below:
A triangle can be the cross section of a rectangular prism, because so the cross-section would be a triangle.
A rectangular prism can be the cross section of a rectangular prism, because the rectangles are the bases there are the ends of the prism. So if we were to slice it parallel to the rectangles.
A trapezoid can be the cross section of a rectangular prism, because the trapezoid is a part of the shape below, the top and bottem base is a form of trapezoid.
An octagon cannot be the cross section of a rectangular prism, because a rectangular prism has only 6 faces and an octagon has 8 sides.
Therefore the correct answer is OCTAGON, which could NOT represent a cross section of this prism.
Hence