A rectangular prism is shaped like a pyramid, but the base is longer in one direction than the other. i.e. a rectangle.
A cross section of a shape is the new face that is formed when you cut a shape into two pieces with a straight cut. The problem tells us that the cross section is parallel to the base of the prism, therefore if the prism is placed on a flat surface with the base facing down, then we would be cutting horizontally.
The new face that is created on the shape after the cut, would be a rectangle due to the shape of the base.
