What is the characteristic of a sphere?

A sphere is defined as a perfectly round three-dimensional shape where every point on its surface is equidistant from its center. This characteristic gives a sphere its unique properties, such as having no edges or vertices, unlike other shapes that may have sharp corners or flat surfaces.

In contrast, shapes with sharp edges and flat surfaces do not conform to the definition of a sphere. For instance, shapes with corners are typically polyhedra, which include cubes and pyramids, but they lack the smooth and continuous surface found in a sphere. Additionally, while hollow shapes can exist, a sphere itself is considered a filled solid structure in most contexts, emphasizing its roundness rather than its hollowness. Thus, the defining feature of a sphere is that it is a solid shape that is perfectly round, making the correct choice the one that highlights this aspect.