Which of the following is NOT a relational database management system?

The correct answer is D, HTML, because HTML (HyperText Markup Language) is not a relational database management system (RDBMS) but rather a standard markup language used for creating web pages and web applications. It is used to structure content on the web, facilitating the display of text, images, and other multimedia elements, but it does not have the capabilities to manage and manipulate data in a relational manner.

In contrast, MySQL, PostgreSQL, and Oracle are all examples of relational database management systems. These systems are designed to store, retrieve, and manage data in a structured format, often using tables that can be related to one another through keys. They support operations like querying, updating, and transaction management, which are central to managing structured data effectively. Consequently, distinguishing HTML from these RDBMS options illustrates the fundamental differences between a markup language and database management systems.