1. The problem asks: Which of the following is a regular polygon? 2. A regular polygon is defined as a polygon with all sides equal in length and all interior angles equal. 3. Let's analyze each option: - A parallelogram: Opposite sides are equal, but adjacent sides and angles are not necessarily equal, so it is not regular. - A rectangle: All angles are equal (90 degrees), but sides are not necessarily equal, so it is not regular. - A rhombus: All sides are equal, but angles are not necessarily equal, so it is not regular. - A square: All sides are equal and all angles are equal (90 degrees), so it is a regular polygon. 4. Therefore, the correct answer is D: square. Final answer: D square