Which of the following is not a valid variable name in JavaScript?

In JavaScript, a valid variable name must adhere to certain rules. Variable names can include letters, digits, underscores, and dollar signs, but they cannot start with a digit. Additionally, certain keywords in JavaScript are reserved for language features and cannot be used as variable names.

The correct answer, "this," is a reserved keyword in JavaScript. It refers to the current context or scope within which a function or object operates. Because "this" is a keyword, attempting to use it as a variable name will lead to a syntax error.

On the other hand, "myVariable," "_myVariable," and "myVariable2" are valid variable names. They follow the naming conventions by starting with a letter or underscore and not conflicting with any reserved keywords. This makes them acceptable identifiers in JavaScript code.