Programmatically Add SharePoint Columns and Formatting theses Column's properties

Hi,
you have done half part what you are trying to do ..
after creating a field with choice type , you need to change the edit format of the choice field type , becuase by default edit format of the choice field type is dropdown
here is the example solution
                SPList list = web.Lists["TaskList"];
                SPFieldCollection fields = list.Fields;
                SPField field = new SPField(fields, "Choice", "Radio");
                fields.Add(field);
                list.Update();
                SPFieldChoice rdField = list.Fields["Radio"] as SPFieldChoice;
                rdField.EditFormat = SPChoiceFormatType.RadioButtons;
                rdField.Choices.Add("Choice1");
                rdField.Choices.Add("Choice2");
                rdField.Update();
                list.Update();
                web.Update();