Which of the following is not a type of database?

All the options listed are types of databases. However, the term “Hierarchical Structure” typically refers to a model rather than a database type itself. While “Hierarchical Database” is a recognized database type (like the others), “Hierarchical Structure” as phrased could be misleading. To clarify:

Answer: d) Hierarchical Structure

Explanation: While “Hierarchical Database” is a database type, the term “Hierarchical Structure” isn’t recognized as a standalone type of database like the others listed (Relational, Network, and Object-Oriented). Thus, it could be viewed as not a type of database.