You can do this using the client side validations included in CSS Form Builder. The idea is that you would need to apply a client side 'required' validation to the first radio option. Next you would apply a client side conditional validation specifying the first radio option as the triggering element then the second radio option as the target. You would choose if trigger is valid then do not validate the target. You will then repeat this process for each radio option you have until you get to the last one, then you would apply any other client side validations that you would need on the page and finally add in a 'show validation errors' at the end.
I have created a simple example of how this type of validation works for you in a form. Please take a look at this example and the help documentation for applying client side validations. If you have any questions about any part of it please let me know and post back with your page you are trying to apply the validations to.
