What is a unique identifier assigned to a transaction by a database management system?

The correct answer is a primary key, which serves as a unique identifier for each transaction or record in a database. A primary key is a crucial aspect of database design, as it ensures that each entry can be uniquely identified, which is essential for maintaining data integrity and facilitating efficient data retrieval.

A primary key must contain unique values, meaning that no two records can have the same primary key value, and it cannot contain null values. This distinctiveness allows a database management system to efficiently manage and distinguish between records, which is integral when performing operations such as updates, deletions, or queries.

In contrast, while a timestamp records the time at which a transaction occurs, it does not serve as a unique identifier. An identifier key and a unique key are not standard terms widely recognized within database terminology, which further clarifies the significance of the primary key in the context of unique identification within a database. The primary key, therefore, stands out as the critical element for uniquely identifying transactions in a structured and reliable manner.