It depends upon the .NET Framework
that you use. Check Jon Skeet’s answer about Versions.
Here is short version of his answer.
C# 1.0 released with .NET 1.0
C# 1.2 (bizarrely enough); released
with .NET 1.1C# 2.0 released with .NET 2.0
C# 3.0 released with .NET 3.5
C# 4.0 released with .NET 4
C# 5.0 released with .NET 4.5
C# 6.0 released with .NET 4.6
C# 7.0 released with .NET 4.6.2
C# 7.3 released with .NET 4.7.2
C# 8.0 released with NET Core 3.0
C# 9.0 released with NET 5.0
C# 10.0 released with NET 6.0