挑战尚未解决，需要进一步研究。

The challenge is ___unresolved, inviting further work.

他童年的创伤至今仍是未解决的心理问题。

The trauma from his childhood remains an ___unresolved psychological issue.

这个问题已经讨论了好几天，但仍然悬而未决。

The issue has been discussed for days but remains ___unresolved.