To understand what the Venn Diagram represents we have to figure out what the two shapes have in common. This is what the left circle represents.
Examining the shapes, we see that the triangle is an equilateral triangle and the rectangle is a square. These are examples of regular polygons, which means all of their sides have the same length. Therefore, the left circle is likely representing regular polygons.
What the circle on the right represents has to be related to the square. For example, it could represent any shape with four vertices, which is a quadrilateral.
Note that this is just one solution to the problem, and there are many more possibilities besides this one.
