Concept:A regular polygon must have all sides equal and all angles equal.Explanation:A pentagon can be regular or irregular, so it is not always regular.A circle is not a polygon because it has no straight sides.An isosceles triangle has only two equal sides, so it is not always regular.A square has four equal sides and four equal angles (each 90∘), making it always a regular polygon.Answer:The correct option is the square (option D).