Which of the following is a feature of relational databases?

Data organized in tables with relationships is a defining characteristic of relational databases. These databases structure information into tables that consist of rows and columns, where each row represents a unique record and each column represents a specific attribute of that record. The relationships between different tables are established through primary and foreign keys, allowing for complex queries and efficient data retrieval.

This method of organization facilitates data integrity and supports relational algebra, making it easier to manage and manipulate the data using structured query language (SQL). The ability to form relationships between tables helps eliminate data redundancy and maintains consistency across the database, which is foundational to the design and functionality of relational databases.

The other options present features or statements that do not define relational databases. Many databases might have free-to-use versions, but that does not specifically pertain to relational databases. Unstructured formats are typically associated with NoSQL databases rather than relational databases, which are fundamentally structured. Additionally, while knowledge of SQL can be beneficial, it is generally expected for users interacting with relational databases, as SQL is the standard language for querying and managing relational data.