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.