What type of attribute is used to establish relationships between entities?

The correct answer is the foreign key. A foreign key is a type of attribute used to create a relationship between two entities in a database. It is defined in one entity and it corresponds to the primary key of another entity. This relationship allows for the linkage of data, enabling queries to connect related information from different entities efficiently.

In a typical relational database model, a primary key uniquely identifies each record in a table, while a foreign key introduces a connection to another table. This setup is crucial in enforcing referential integrity, meaning that the data remains consistent across the database, since the foreign key must match a primary key in the referenced entity or be null if no relationship exists.

Using a unique attribute, while valuable for identifying a specific record within the same entity, does not establish a relationship between two separate entities. Reference attributes serve other purposes, often acting as pointers or markers but are not specifically meant for creating relationships in the same way that foreign keys do.