Answer: B) Primary key.
Explanation: In Microsoft Access 2019, the AutoNumber data type is used to create a unique identifier, or primary key, for each record in a table. The AutoNumber data type automatically generates a unique number for each new record, making it ideal for use as the primary key. This ensures that each record in a table has a unique identifier, which is important for maintaining data integrity and ensuring the accuracy of relationships between tables. A primary key is a key field that is used to uniquely identify each record in a table, and is often used to establish relationships between tables.