Answer:
a. backdoor
backdoor (also called a trapdoor) is a programming routine built into a system by its designer or programmer. It enables the designer or programmer to bypass system security and sneak back into the system later to access programs or files.
Explanation:
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This is about identifiers in a record referring to other records.
You can have many to one, one to one, many to many.
E.g., if you have two tables, Authors and Books, then a book record could have a reference to an author record. Since an author can write many books, this would be a many-to-one relationship.