Thanks for reaching out to the Community! This a brand new issue, we were made aware just yesterday that contacts who are using an iPhone are unable to see any options for questions that use a dropdown menu. (checkbox multiple choice options work, but selecting from a dropdown menu does not.) At this point it seems to be just iPhones, and we have not had any reports of this happening on Android.
Our Development team is working to get this fixed, and I have tracked this on your behalf. We will send you an email once the issue has been resolved.
At this time the listed workaround is to use a different style of question that does not use dropdown menus.