What defines a primary key in a relational database?

A primary key is a fundamental concept in relational database design. Its main function is to uniquely identify each row in a table, ensuring that no two rows can have the same primary key value. This uniqueness is crucial for maintaining data integrity and allowing for efficient data retrieval.

When a primary key is established, it creates a distinct identification for each record, enabling relationships between tables where foreign keys might reference this primary key. This characteristic is what enables databases to efficiently organize and access data without ambiguity.

The other options do not accurately reflect the true nature of a primary key. The concept of duplication contradicts the essence of a primary key's role in ensuring uniqueness. Allowing for multiple columns can be true in the case of composite primary keys, but it does not define what a primary key is. Similarly, a primary key must not contain null values, as this would prevent it from fulfilling its purpose of uniquely identifying records. Thus, B is the correct answer as it directly defines the primary key's role in relational databases.