Which of the following is NOT an open-source relational database?

Microsoft SQL Server is not an open-source relational database. It is a proprietary software developed by Microsoft and requires a license for use. This means that the source code is not made available to the public, and the software is controlled and maintained solely by Microsoft, limiting users' ability to modify or distribute it freely.

In contrast, PostgreSQL, MySQL, and SQLite are all open-source databases. They allow users access to their source codes, enabling modifications, distributions, and participation in their development by the community. This open-source nature fosters collaboration and innovation, allowing for rapid advancements and extensive support through community contributions.