The correct answer is it is a figure with at least 3 straight sides. When you think of a square or a triangle they both have at least 3 sides that are straight. A circle doesn't have any straight sides so it is not a polygon.
You can prove that the other answer is wrong because of two things:
The definition of a polygon is a plane figure with at least three straight sides.
Also, when you think of most shapes what do they all have in common? The have at least three straight sides