Which of the following is NOT a SQL database?

The correct choice indicates that MongoDB is not a SQL database. This is because MongoDB is designed as a NoSQL database, which means it does not utilize the structured query language (SQL) for managing and querying data. Instead, MongoDB stores data in JSON-like documents, which allows for flexible and dynamic schemas. This contrasts with SQL databases, which use structured tables with predefined schemas.

In terms of database types, SQL databases such as PostgreSQL, MySQL, and SQLite use SQL to perform operations like querying, updating, inserting, and deleting data, relying on schema-based organization of data. This structured format is not inherent in MongoDB's design, where the emphasis is placed on scalability and the ability to handle unstructured data. As a result, MongoDB serves a different purpose and is best suited for use cases requiring high flexibility, accommodating rapidly evolving data across various applications.