choice of different version of dll in our project

Usually you don't need to add explicit reference to the shared assembly to the project because it never gets copied and deployed with your project.

Refer this thread for detailed information - http://stackoverflow.com/questions/14891422/referencing-dll-from-gac-in-visual-studio

To refer an assembly with perticulat version - you might need to do changes in your application's configuration file and mention it's fully qualified name or use the binding redirect.

http://msdn.microsoft.com/en-us/library/7wd6ex19(v=vs.110).aspx
http://stackoverflow.com/questions/16578819/understanding-a-csproj-assembly-reference

actually i created private assembly and changed into shared assembly with multiple version , and now i want to add any specific version of assembly in my project, this is a problem.
i don't understand how to resolve it.........???

What do you mean by 'choose'? any specific .NET assembly you are talking about?
are you trying to add an assembly with two different versions and need help in it or trying to set the assembly version of your project?

You can refer this article which is a great source of information about assemblies and their versioning.
http://msdn.microsoft.com/en-us/library/51ket42z(v=vs.110).aspx