Which of the following is a regular quadrilateral?

Which of the following is a regular quadrilateral?
  1. Rectangle
  2. Rhombus
  3. Square
  4. Parallelogram

Solution

A square has all $4$ sides equal AND all $4$ angles equal ($90^{\circ}$), so it is a regular quadrilateral. A rectangle has equal angles but not equal sides; a rhombus has equal sides but not equal angles.

Asked in: IMO

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