Behaviour of radio buttons

This seems like an elementary question, but I can't find an answer. How do I configure a multiple choice field so that just one value can be selected? Even if I choose "Radio buttons", more than one value can be chosen. Alas, I am old enough to remember car radios with real buttons that you used to change stations. One you pressed one button, a previously selected button popped out. Obviously you couldn't liste to more than one radio frequency at a time. I want my buttons to behave in the same way.

Thanks