Enum FeedbackType

// Open a form by ID and add a new list item.
var form = FormApp.openById('1234567890abcdefghijklmnopqrstuvwxyz');
var item = form.addListItem();
item.setTitle('Do you prefer cats or dogs?');
// Set "Dogs" as the correct answer to this question.
item.setChoices([
  item.createChoice('Dogs', true),
  item.createChoice('Cats', false)]);
// Add feedback which will be shown for correct responses; ie "Dogs".
item.setFeedbackForCorrect(
    FormApp.createFeedback().setDisplayText("Dogs rule, cats drool.").build());