ADD SPFieldType.Choice, SPFieldChoice with SPChoiceFormatType.RadioButtons

 using Microsoft.SharePoint;

 using System.Collections.Specialized;

 public void OnActivated(SPFeatureReceiverProperties properties)

 {

     SPWeb web = (SPWeb)properties.Feature.Parent;

     string url = new Uri(web.Url).AbsolutePath;

     SPDocumentLibrary docLib = (SPDocumentLibrary)web.GetListFromUrl("Shared Documents/Forms/AllItems.aspx");

     // Create collection with choices.

     StringCollection categories = new StringCollection();

     categories.AddRange(new string [] {"Choice A", "Choice B", "Choice C"} );

     // Create new field in the project documents list.

     docLib.Fields.Add("MyNewField", SPFieldType.Choice, true, false, categories);

     // Set additional settings of the new field.

     SPFieldChoice fieldChoice = (SPFieldChoice)docLib.Fields["MyNewField"];

     fieldChoice.DefaultValue = "Choice A";

     fieldChoice.EditFormat = SPChoiceFormatType.RadioButtons;

     // Save settings.

     fieldChoice.Update();

     docLib.Update();

 }