I was wrong with "Which of you is more stupid," because DL wanted "Which ONE of you..." In other similar sentences, the "one" part hasn't been needed, so I don't understand why it's needed in this sentence. In case it really isn't needed (hope!), I've reported it.
The word bank should contain all the tiles needed to form the "main" translation shown above, plus some "decoy" words. So it sounds like there may be something strange happening on the system side. If that's the case, it is not something that the course team can fix. You may need to file a bug report directly with Duoling; info is available here: https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-.