Which of the following is NOT a valid identifier in C#?

Answer: Option B

Identifiers in C# are names given to variables, methods, classes, etc.

Valid identifiers in C# can include letters (a-z, A-Z), digits (0-9), and underscores (_), but they cannot start with a digit.

Option A: _myVariable - This is a valid identifier because it starts with an underscore and follows the rules.

Option B: 2ndVariable - This is an invalid identifier because it starts with a digit, which is not allowed in C# identifiers.

Option C: My_Variable - This is a valid identifier as it follows the rules.

Option D: myVariable_2 - This is a valid identifier as it follows the rules.

Therefore, the correct answer is Option B: 2ndVariable.