Answer: C) Is the material being shared copyrighted?
Explanation:
This is the most important and indeed the only thing that matters when it comes to determining whether a specific instance of file sharing is legal.
If the information is Copyrighted that means it is protected from unauthorized sharing by law. Sharing it therefore without authorization is considered illegal because the law is literally being broken.
If the information is not copyrighted then it can be shared at will, made money off of, shared on sites that regularly share illegally and the like. But once it is protected by law, it's best not to share without authorization.