Which of the following is an example of a polygon?

A polygon is defined as a closed figure made up of straight line segments that are connected end to end. These line segments are referred to as the sides of the polygon, and the points where they intersect are the vertices.

In this case, a triangle meets the criteria for being a polygon as it is formed by three straight sides and three angles, making it a simple polygon. It has vertices where its sides meet, fulfilling the requirement of a closed shape made only from line segments.

In contrast, a circle is not a polygon because it has a curved boundary and does not consist of straight lines. A sphere, being a three-dimensional object with a curved surface, also does not fit the polygon definition. Lastly, a curve by itself does not define a polygon because it can be open-ended and lacks the necessary straight sides and vertices that characterize polygons.