Deletes one option from a dropdown, listbox, radio, or checkbox custom field, leaving the field and its other options in place. The id is a field option id from the fieldOptions array of GET /fields or GET /fieldOptions, never a custom field definition id; deleting the whole field is DELETE /fields/{id}, which also destroys all contact values stored under it. Deleting an option does not update contacts: a contact whose saved value is the deleted option keeps that value until it is changed. 404 if no option has this id.