Here are the explanations for each identifier:
1. break$: This is a valid identifier because it does not start with a digit and does not use any reserved keywords by itself. Special characters like $ are allowed in identifiers.
2. _9Number: This is a valid identifier. Identifiers can start with an underscore, and the rest of the name can include digits and letters.
3. middle_Name: This is a valid identifier. It follows the rules, where underscores are allowed, and it does not begin with a digit or use reserved keywords.
4. default: This is not a valid identifier because default is a reserved keyword in Java.