What must a foreign key be related to in a relational database?

A foreign key must be related to an existing primary key value in a relational database. This relationship is fundamental to maintaining referential integrity within the database. When a foreign key is established in a table, it creates a link to the primary key in another table, thereby ensuring that the values in the foreign key column correspond to, and thus are valid references to, the primary key of the referenced table.

This connection allows for the enforcement of rules that ensure the data remains consistent across related tables. If a foreign key attempts to reference a primary key value that does not exist, the database can reject that entry, preventing orphaned records and maintaining the accuracy of the overall data structure.

The other options are related to database design but do not accurately describe the requirement for a foreign key. A table name refers to the structure of the database itself, while a data type definition pertains to how data is formatted in the database. An index value involves optimizing data retrieval but is not a requirement for establishing a foreign key relationship. Thus, the requirement for a foreign key is fundamentally tied to valid and existing primary key values.