Which of the following integrity constraints ensures that every non-null foreign key value must reference an existing primary key value in another table?

Referential integrity enforces a relationship between two tables by ensuring that a foreign key in one table matches a valid primary key in another table. This constraint guarantees that the data is consistent and prevents "orphan" records. For example, in an "Orders" table with a foreign key referencing a "Customers" table, referential integrity ensures that every order must belong to an existing customer. If a foreign key value does not match any primary key value in the referenced table, the database system will reject the operation, ensuring data consistency. Why Other Options Are Incorrect:

• Option A: Entity integrity relates to primary keys, ensuring they are unique and not NULL. It does not address relationships between tables.
• Option C: Domain integrity enforces the validity of data types and permissible values for attributes, such as ensuring an "age" column only contains integers.
• Option D: Key integrity is not a formal term in relational database theory. It may be confused with primary or candidate key constraints but is not relevant here.
• Option E: Tuple integrity is not a standard concept in relational databases, making this option invalid.