In my opinion pending sounds better than undone
Pending would be a better word than "undone," although it's commonly used for payments or revisions (in business).
Personally I'm leaning towards Solved as it is both common and as vague as possible.
Also "Solved" sounds better than "undone." But you should ask yourself (a good way to see if something flows or not is to hear yourself saying it out loud, and not just in your head) which one sounds better?:
"This thread has been solved"
"This thread has been resolved"
"This thread has been done"
"This thread has been answered"
"The thread is pending" (pending what? an answer?)
Now, which one of those above did sound better when you said it out loud?