Question Drag n Drop in specific area

Hey, atm i am working on a drag n drop game, and i got some regions where you can spawn units. Those units should only be allowed to move inside of the circle, of there own country (like in the picture).
So lets say i am spawning a unit from the country with the blue circle. Those units should only be allowed to be drag n droped inside the blue circle. If i then drop the unit from the blue circle inside the inventory of the white circle, those units should then be only allowed to move inside the white circle.
I hope i could explain in to you good enough cuz english is not my first language .

 

are those UI elements or sprites or other gameobjects in scene, do they have colliders?

if it has 2d collider, could try raycasting, if mouse hits that blue collider, then can place object.
https://docs.unity3d.com/ScriptReference/Physics2D.Raycast.html

if that shape was perfect circle, would be easy to calculate distance from center also.
https://docs.unity3d.com/ScriptReference/Vector2.Distance.html