I created a PDF in Adobe Designer 7.0 with the usual text fields.

I also added three RadioButtonList controls.

Populating the PDF's text fields was trivial, but selecting a PDF radio button was not.

I checked the iText (Java) documentation and found instructions. 

Here's how in C#: 

/* myRadioButton1 is what I named the Radio Button control in Adobe Designer. The "1", "2" etc. are the value automatically assigned by Adobe Designer. */

fields.SetListOption("myRadioButton1", new String[] { "1", "2", "3", "4", "5", "6" }, null);

/* this selects the second radio button. */

fields.SetField("myRadioButton1", "2");

I hope this helps someone else.

tags: RadioButtonList Radio RadioButton SetListOption input type="radio"