Which of the following is a free and open-source relational database?

MySQL is a free and open-source relational database management system (RDBMS), which means that it can be freely downloaded, used, modified, and distributed by anyone. This accessibility makes it a popular choice for both small and large applications, as it offers robust features and the support of a large community.

In contrast, Microsoft Access, Oracle Database, and MSSQL Server are proprietary software. Microsoft Access is designed for small-scale applications and lacks the scalability of MySQL. Oracle Database and MSSQL Server are enterprise-level databases, requiring licenses for use, which adds to their cost and limits their free usage compared to MySQL.

By being open-source, MySQL also benefits from a collaborative development model that allows for rapid updates and enhancements, contributed by users and developers around the world. This model ensures that MySQL continues to evolve to meet the needs of its users while being accessible to anyone interested in implementing a relational database system.