I have searched the most obvious sources but can't find the ternary operator listed as being introduced in a specific version. (I need to know this information for a historical reference).
It was introduced at the beginning, in C# 1.
If you want an authoritative source, the Wikipedia page has links to the specifications. The ECMA Spec for C# 1.0, 1.1 and 1.2 says:
14.12 Conditional operator
The
?:
operator is called the conditional operator. It is at times also called the ternary operator.conditional-expression: conditional-or-expression conditional-or-expression ? expression : expression
... and so on.