To determine which shape has a circular cross-section when the cross section is taken parallel to the base, let's analyze each option:
A. Cone - A cone has a circular base. If you take a cross-section parallel to this base at any height, you will indeed get a circle.
B. Dodecahedron - A dodecahedron is a polyhedron with twelve flat faces, each a regular pentagon. No cross-section of a dodecahedron will yield a circular shape since the sides are all polygons, and there are no curved surfaces.
C. Rectangular Prism - A rectangular prism has rectangular faces. If you take a cross-section parallel to its base, you will always end up with another rectangle, not a circle.
D. Octahedron - An octahedron is a polyhedron with eight equilateral triangle faces. Similar to the dodecahedron, it does not have any curved surfaces, so no cross-section will result in a circle.
Given these descriptions, the shape that has a circular cross-section when taken parallel to the base is:
A. cone
Thus, the best answer is:
A
