Which is the default Scripting Language of ASP (server-side)?

I'm not sure if there's a default language, but I think that all .NET languages (C#, VB.NET, ...
etc) will eventually be converted into Common Intermediate Language (CIL) which then will be executed in any environment that supports Common Language Infrastructure (CLI) (.NET runtime for Widnows or cross-platfrom Mono Project).
Believe it or not, you can use VB.NET classes in an ASP.NET project written in C#.
Hope that helps :)

VBScript is the default scripting language for ASP.

in Classic ASP, VB Script is a default scripting language.

There is no default language when it comes to ASP.NET, you get to pick your choice when you create new classes, pages, etc.
If you mean the most popular, I'd believe that to be C#.
Also note that these are not "scripting" languages, they are all compiled.

VBScript is the default scripting language for ASP.