Which of the following identifiers is an invalid variable name?

In most programming languages, variable names must adhere to specific rules. Typically, these rules include:

1. The name must start with a letter (uppercase or lowercase) or an underscore (_).
2. The name can only contain letters, digits, and underscores.

Given these rules, let’s evaluate the provided identifiers:

1. 0_: This is an invalid variable name because it starts with a digit.
2. 0: This is a valid variable name. It starts with an underscore and contains only letters and underscores.
3. _0: This is a valid variable name. It starts with an underscore and contains only letters and underscores.
4. ___: This is a valid variable name. It starts with an underscore and contains only underscores.

So, the invalid variable name is 0_.