Polygons
Both rectangles and squares are polygons. A square is a special case of a rectangle.
Squares are polygons, but not all polygons are squares.
the answer is a square
A set of polygons might have such shapes in it. Certainly the set of all polygons does. So does the set of all polygons with fewer than 5 sides. Many other examples could be given. I am avoiding the use of the word "group", because it has a technical meaning in modern algebra.
they are all polygons
Regular Polygons. eg - Square is a regular figure, but a trapezium is not.
A square is a special type of polygon.
They are polygons.
They are both polygons.
they are both polygons
Yes.