Answer: The correct option is
(A) A geometric shape with three or more sides.
Step-by-step explanation: We are given to select the best definition of a polygon.
We know that
a polygon is a geometric shape having number of sides greater than or equal to three.
For example,
(a) A triangle is a polygon having three sides.
(b) A quadrilateral is a polygon having four sides.
(c) A pentagon is a polygon having five sides.
Three polygons, a triangle, a quadrilateral and a pentagon are shown in the attached figure below.
Thus, option (A) is CORRECT.