Most FileMaker users would probably be unaware that you can select multiple options in a field displayed as a radio button. Generally radio buttons are to force a single choice on the user, but if you hold down shift you can actually still select multiple options. But there is an easy way to force the field to be single option only. Just add an auto enter calc to the field definition with :

GetValue ( Self ; 1 )

Which will mean when you try to select a second option, it will always revert to the first option you select.

You can also extend this to also force the user to choose one option from the available choices, and restore a default if none have been chosen by adding to the calc :

Let ( new = FilterValues ( "ValueList" ; Self ) ; Case ( IsEmpty ( new ) ; "Default" ; GetValue ( new ; 1 ) ) )

You’d need to replace “ValueList” and “Default” with the appropriate values, either generated from the file itself or even just manually coded.