What specific .NET version does unity support?

I read here Unity Script Reference – Mono Compatibility that we can use .NET 2.0 class libraries. Is that correct? and if so, then, do we need to use C# 2.0 or we can use C# 3.0 or 4.0? I dont understand if there is a relation between the C# version and the .NET libraries that unity support. Please explain me.
Sorry for my bad English, I`m Chilean

Unity uses none of those. It uses Mono, which is something in between .NET 2.0 and 3.5. You can check compatibility here.