According to the properties of a parallelogram.
Its opposite angles are equal and its opposite sides are also equal.
Now for a parallelogram to be a rectangle,
All its angles must be 90°.
But we also want the parallelogram to be a rhombus, which means all its sides must be equal.
Now any rectangle whose all sides are equal becomes a square.
And so parallelogram which is both a rectangle and a rhombus is a SQUARE.