Which of the following polygons can be classified as regular?

A regular polygon is defined as a polygon that is both equilateral (all sides are of equal length) and equiangular (all angles are of equal measure).

In the context of the given question, a square can be classified as a regular polygon because it has four sides of equal length and all interior angles are equal to 90 degrees. The symmetry and uniformity of a square make it a prime example of a regular polygon.

While it is possible for triangles and pentagons to be regular if they meet the criteria of equal sides and angles, the question specifies a definitive answer, which in this case is the square, commonly recognized for its regularity. The star shape, on the other hand, typically does not meet the criteria for a regular polygon due to its varying side lengths and angles.

Thus, the classification of the square as a regular polygon is correct because it exemplifies the defining characteristics of regularity in polygons, making it a suitable answer for the question.