The key word here is ‘perpendicular’. Since they say that the cross section is perpendicular to the rectangular base, the cross section will still be a rectangle, just smaller, as long as it does not pass through the top vertex.
Therefore, the answer is (b) a rectangle.
Just a side note: IF the cross section is perpendicular to the base and it DOES pass through the top vertex, then it will have no shape at all, since it will just be a single point.
