we know that
The base of a rectangular prism is a rectangle
so
when a rectangular prism is being sliced by a plane parallel to its base
the shape of the cross section formed is a rectangle
see the attached figure to better understand the problem
In this problem
the base of the rectangular prism is the rectangle ABFE and the cross section formed by a plane parallel to its base is the rectangle shaded in the figure
therefore
the answer is
A rectangle
