Going off of how Toby's last forum project went[1], most things that aren't essential to the project are left open to implement as plugins. esoTalk does have a plugin that lets conversation authors mark one reply as an answer, so I think it's reasonable to guess that one will eventually appear for Flarum.

[1]: http://esotalk.org

We built Flarum to be highly extensible, so something like this could easily be implemented! Eventually we'll probably make an official Q&A extension too. If you want to learn more about Flarum's extensibility, check out http://flarum.org/docs/extend/.