What is a primary key in a relational database?

A primary key in a relational database serves the crucial function of uniquely identifying each record within a table. This uniqueness ensures that no two rows have the same primary key value, allowing for efficient data retrieval and the establishment of relationships between tables. When you mention that a primary key is a "unique value used to reference another key," it highlights its role in the relational model. When two tables are connected, the primary key from one table can act as a foreign key in another table, linking them together in a meaningful way. This ensures data integrity and helps maintain consistent relationships across the database.

The other options do not accurately describe the function of a primary key. While encryption and data backup are important aspects of data management, they pertain to security and recovery rather than uniquely identifying records. Similarly, while an index can improve data retrieval speed, it is not necessarily the same as a primary key, which specifically serves to maintain unique identifiers for records within the database structure.