Which of the following is not a type of database?

The correct answer is d) Hierarchical Structure.

Explanation: While “Hierarchical” can refer to a type of database model, “Hierarchical Structure” itself is not a recognized type of database. The primary types of databases include Relational, Network, and Object-Oriented databases. Each of these has its own structure and methods for organizing and accessing data. If you have more questions about databases or related topics, feel free to ask!