Each of these are closed figures formed by straight line segments. Think of fencing in an area (not necessarily a rectangle) using various straight fence sections. The length of each section does not have to be the same.
Side notes:
Choice A is not a polygon because the figure is not closed.
Choice D is not a polygon because it is not composed of line segments only. Choice D is an ellipse instead.
Choice E is not a polygon due to the curved portion.
