Answer:
U ={ Parallelograms}
A= { Parallelogram with four congruent sides}={ Rhombus,Square}
B ={ Parallelograms with four congruent angles} ={ Rectangle, Square}
So, AB= { Square}
So among all the parallelograms "Square" is the only parallelogram which has all congruent sides as well as all congruent angles.