Which of the following is a regular polygon?
A regular polygon is a polygon which is equiangular i.e. all the angles are equal and all the sides are equal.
An equilateral triangle is a regular polygon. No other triangle is a regular polygon.
A rhombus has equal sides but not equal angles.
A rectangle has equal angles but not equal sides. Hence, they are not regular polygons.
A square has equal sides and equal angles. Hence, it is a regular polygon.