C# code to add Choice Field column to a SharePoint List Programmatically

/* get the newly added choice field instance */

SPFieldChoice chFldGender = (SPFieldChoice)lst.Fields["Gender"];

/* set field format type i.e. radio / dropdown */

chFldGender.EditFormat = SPChoiceFormatType.Dropdown;

/* set the choice strings and update the field */

chFldGender.Choices.Add("Male");

chFldGender.Choices.Add("Female");

chFldGender.Update();

/* finally update list */

lst.Update();