Which of the following is not a valid variable name?

A valid variable name in programming must adhere to certain conventions and rules. In most programming languages, variable names can include letters (both uppercase and lowercase), numbers, underscores, and certain special characters, but they cannot start with a number or include other invalid characters.

The name "variable_2" is valid because it follows the standard naming convention by starting with a letter and including an underscore, which is acceptable. Similarly, "variable$2" is also a valid variable name as the dollar sign is permitted in many programming languages.

On the other hand, "variable-2" is not valid because it contains a hyphen, which is not allowed in variable names; the hyphen is interpreted as a minus operator in many languages, making it ambiguous. Likewise, "variable+2" is invalid because the plus sign is also a mathematical operator rather than a permitted character for naming variables.

Thus, the reason "variable-2" is identified as not a valid variable name is that the hyphen disqualifies it under standard variable naming rules, which is consistent across many programming languages.