If you want to study a single section of a list, you can do one of two things:
1) Do it on the website, where single section study lives.
2) Copy the section content into a new, single-section list, then study that list by itself on the iOS app.
We aren't going to implement single-section study in the iOS app after seeing what happened with it on the web (not many people used it, and those that did mostly did so out of confusion and then emailed us all the time until we made it hard to find), but it is still there on the web, at least.