This option is back now. You can now mark your threads as RESOLVED, by pulling down Thread Tools and clicking on "Mark Thread as RESOLVED". Now this has been improved, I think. Because in old version, in every thread this option was 'shown'. But in this version, only those threads that you have created will have this option. And also, only unresolved threads would have this option. Once you mark the thread RESOLVED, that option wouldn't be shown on that thread. If you wish to mark it as unresolved(later after marking it as RESOLVED), you could edit your thread (go Advanced Mode) and remove the "[RESOLVED]" from the title. Now, it will bring back the "Mark Thread as RESOLVED" option for that thread.

This is a nice improvement. But I think, it would have been a lot more better to have a "Mark thread as UNRESOLVED" option also. So that within a single click, we could mark it as unresolved. But it is not a compulsory one, because there would be lesser chance to mark a thread as "unresolved".