Which figure has a three-dimensional structure but is not defined by flat surfaces?

The correct answer is a cylinder. A cylinder is a three-dimensional geometric shape that consists of two parallel circular bases connected by a curved surface. This curved surface means that the cylinder does not have flat surfaces forming its entire shape, distinguishing it from other three-dimensional figures like pyramids and polyhedra that rely primarily on flat surfaces.

In contrast, a polygon refers to a two-dimensional shape made up of straight line segments and flat surfaces, while both pyramids and other three-dimensional figures, such as spheres, similarly rely on flat surfaces for their structural components. A pyramid has a flat base and triangular sides that converge at a point, while a sphere is perfectly round and has no flat surfaces at all, but is not classified under surfaces like the cylinder. A cylinder stands out as it embodies a blend of flat and curved geometry, establishing its unique three-dimensional structure.