Which of the following shapes is an example of a polygon?

A square is considered a polygon because it is a two-dimensional geometric figure that meets the criteria defining polygons. Specifically, a polygon must have at least three sides, be closed (having no openings), and have straight edges. A square fulfills all these requirements as it has four equal sides, four angles, and encloses a region in space.

In contrast, a circle is not a polygon because it has a curved boundary and does not consist of straight edges. Similarly, a sphere is a three-dimensional shape that does not fit the definition of a polygon due to its continuous curved surface. Finally, while a pyramid does have a polygonal base and triangular faces, it exists in three dimensions, which places it outside the category of polygons, typically reserved for flat, two-dimensional shapes.