Which of the following is not a valid identifier in Java?

In Java, valid identifiers must follow specific rules. Identifiers can consist of letters (both uppercase and lowercase), digits, underscores, and dollar signs. However, they cannot start with a digit.

The given identifier "2ndVariable" begins with a digit, which makes it invalid according to Java's naming conventions for identifiers. Identifiers must begin with either a letter (A-Z or a-z), an underscore (_), or a dollar sign ($), which is why the other choices (variable_name, variableName, and $variable) are considered valid. They either start with a letter, underscore, or dollar sign, all of which are acceptable starting characters for identifiers in Java.