/*
    Add this script to a form.
    Go to Edit > Current Project Triggers
    Add the addOption function to the On Submit trigger
    Fill the form once.
    Go to View > Logs
    Change the id 00000000 for the one that was logged.
*/
function addOption(e) {
  try {
    var form = FormApp.getActiveForm();
    var formResponse = e.response;
    var itemResponses = formResponse.getItemResponses();

    for (var j = 0; j < itemResponses.length; j++) {
      var itemResponse = itemResponses[j];
      var id = itemResponse.getItem().getId();
      Logger.log('id ' + itemResponse.getItem().getId()); // 00000000

      if ( id == 00000000) { // Replace with your own id after logging it first
        var item = form.getItemById(id).asMultipleChoiceItem();
        var choices = item.getChoices();
        var add = true;
        var answer = itemResponse.getResponse();

        for (var k = 0; k < choices.length; k++) {
          if (answer == choices[k].getValue()) {
            add = false;
          }
          Logger.log(itemResponse.getResponse() + ', ' + choices[k].getValue());
        }
        if (add) {
          choices.push( item.createChoice(itemResponse.getResponse()) );
          item.setChoices(choices);
        }
      }
    }
  } catch (err) { Logger.log(err); }
}