When you add a question to a job's application form, the answer types say what applicants will see: Drop-down menu (pick one), Multiple choice (pick one) or Checkboxes (pick any).
The Choices box stays faded out until you pick one of those types.
Clearer choice questions on application forms
Choice questions on a job's application form are now called Drop-down menu, Multiple choice and Checkboxes, and the Choices box only turns on for them.