Which of the following figures is not classified as a polygon?

A polygon is defined as a closed, two-dimensional shape composed of a finite number of straight line segments that connect to form a closed figure. The key characteristics of polygons include having straight edges and being flat (two-dimensional).

The first three options—a square, rectangle, and triangle—are all polygons because they consist of straight lines, form a closed shape, and lie in a two-dimensional plane. Specifically, a square has four equal sides and right angles, a rectangle has opposite sides equal and four right angles, and a triangle has three sides and three angles.

In contrast, the last option, the cone, is not a polygon. While it is a three-dimensional shape, it contains curved surfaces and only has a circular base instead of straight edges. Thus, the distinction lies in the dimensions and the type of edges—polygons must have straight sides and be two-dimensional, which the cone does not satisfy.