What defines a primary key in the context of a CDT?

A primary key in the context of a Complex Data Type (CDT) is defined as a field that uniquely identifies a row of data. This uniqueness is essential because it ensures that each entry in the data table can be distinctly referenced without ambiguity.

When designing a database, having a primary key helps maintain data integrity, as each record can be retrieved, modified, or deleted based solely on this unique identifier. It plays a critical role in establishing relationships between different datasets and is a foundational concept in database management.

Other options like a field that must be filled out or one that is optional do not intrinsically define the uniqueness aspect or the role of a primary key. Similarly, the idea that a field can be changed regularly does not pertain to its function as a primary key; rather, primary keys should remain constant for accurate data integrity and identification throughout their usage.