Answer: C. A rectangle.
Explanation:
1. By definition, a cross section is the intersection of a plane and a body of three dimensions.
2. A pentagonal prism has a pentagon as its base and its lateral faces are rectangles.
3. If the pentagonal prism is cut by a plane perpendicular to the base, this means that the plane is parallel to lateral faces, then, the shape of the cross section that is formed is also a rectangle.
4. The answer is the option C.