Mono version of unity

Hi, I searched in document and did not find any info about version of Mono unity uses. I really appreciate that anyone tell me which version of Mono unity 5.x uses?? or which version .Net it we can use??
Thanks in advance.

As far as I know, it’s .NET 3.5

Unity currently uses Mono 2.6. We essentially support .Net 3.5 and almost all of C# 4 (the language). Note that the framework is distinct from the language.

However, we also have to deal with all sorts of platform restrictions which complicates the matter. For example many things will never work under AOT-compiled platforms, some platforms are security restricted and run a subset of the full .Net profile. Other platforms are even more constrained (like WebGL) and cannot even use things like threads or .Net sockets.