Which of the following is NOT a valid variable name in Python?

In Python, variable names must adhere to specific rules to be considered valid. A valid variable name must start with a letter (a-z, A-Z) or an underscore (_), and can be followed by letters, digits (0-9), or underscores.

The variable name containing "2nd_var" begins with a digit, which is not permitted in Python. This is why it is identified as an invalid variable name.

The other options start with a letter or an underscore and follow the rules for valid variable naming. "var_name" contains an underscore and is valid, "varName" follows camel case conventions and is also valid, and "VAR123" begins with letters and includes numbers, making it a valid identifier too. Thus, the option "2nd_var" is the only one that does not meet the criteria for a valid variable name in Python.