What is a 3D shape formed by flat plane surfaces called?

A three-dimensional shape formed by flat plane surfaces is known as a polyhedron. This type of shape is characterized by having solid faces that are polygons, which means each face is flat and each edge meets at vertices. The common examples of polyhedra include cubes, tetrahedrons, and dodecahedrons, all illustrating that a polyhedron can take various forms but is distinctly defined by its flat surfaces.

In contrast, the other options do not meet the criteria of consisting solely of flat surfaces. A pyramid, for example, has a polygonal base and triangular faces that converge to a single point, introducing curved surfaces if discussing pyramids with rounded bases. A cylinder has two flat circular bases connected by a curved surface, and while it does contain flat elements, it is not formed entirely of flat plane surfaces. Lastly, a sphere is a completely round object with no edges or flat surfaces at all, making it entirely smooth. Thus, the defining feature of a polyhedron establishes it as the correct answer in this context.