To answer this question, we need to understand the rules for legal identifiers in programming languages.

In general, legal identifiers must follow these rules:

1. They can only contain letters (both uppercase and lowercase), digits, and underscores.
2. They cannot start with a digit.
3. They cannot contain special characters such as +, -, *, /, etc.

Let's go through each option to determine which ones are not legal identifiers:

A. 1alpha - This option is not a legal identifier because it starts with a digit.

B. _abcd - This option is a legal identifier because it starts with an underscore and contains only letters, digits, and underscores.

C. xy+abc - This option is not a legal identifier because it contains the special character '+'.

D. transient - This option is a legal identifier because it contains only letters.

E. account-num - This option is not a legal identifier because it contains the hyphen '-'.

F. very_long_name - This option is a legal identifier because it contains only letters, and underscores.

Therefore, the correct answers are A, C, D, and E.

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