Final answer:
The identifiers 'theDog', 'recycling', 'DVD_ROM', and 'Elizabeth_the_2nd' are valid, while 'all - In - One', 'const', '2morrow', and 'page#' are invalid due to containing spaces, hyphens, starting with a digit, or including invalid special characters.
Step-by-step explanation:
The task is to mark the following identifiers as either valid or invalid:
- theDog: Valid
- all - In - One: Invalid because it contains spaces and hyphens.
- const: Reserved keyword, so it's invalid as an identifier.
- recycling: Valid
- DVD_ROM: Valid
- Elizabeth_the_2nd: Valid
- 2morrow: Invalid because it starts with a digit.
- page#: Invalid because it contains a special character not allowed in identifiers.
In most programming languages, a valid identifier must start with a letter or an underscore and may be followed by letters, digits, or underscores. It should not be a reserved keyword, contain spaces, hyphens, or any special characters (other than underscores).