A primary key in a relational database table is used to uniquely identify each record in that table. It enforces entity integrity by ensuring that each row can be distinguished from every other row. A primary key must contain unique values and it cannot contain NULL values. This guarantees that every record is distinct and can be retrieved, updated, or related to other records efficiently.
The primary key serves as the central reference point when creating relationships between tables through the use of foreign keys, thus enabling referential integrity within the database. Without a primary key, it would not be possible to maintain consistent and reliable relationships across different tables.
Option A is incorrect because connecting tables to external systems is not the role of a primary key; this function is handled through integration tools or middleware.
Option B is incorrect because managing user access is controlled by database access control mechanisms, not primary keys.
Option D is incorrect because improving storage efficiency is related to data normalization, indexing, and storage optimization, not primary key assignment.
Therefore, the correct function of a primary key is to identify each record in a table uniquely, ensuring data integrity and reliable referencing across the database.