Which of the following is not a valid variable name?

The reason the first choice is not a valid variable name is that variable names must adhere to specific naming conventions in programming languages. In most programming languages, a variable name cannot start with a digit. It can only begin with a letter (either uppercase or lowercase) or an underscore. This prevents confusion with numeric literals and ensures that variable names are easily distinguished.

In contrast, the other choices follow the accepted rules for naming variables. They start with letters (either 'o' or 'v') followed by other characters, which can include additional letters, numbers, and underscores. Therefore, only the first choice violates the fundamental rules governing variable naming, making it invalid.