The correct answer is C. MySQL.
MySQL is a free and open-source relational database management system (RDBMS) that is widely used and popular among web developers. It is easy to use and has a large community of users and developers. MySQL is also highly scalable and can be used to power large websites and applications.
Microsoft SQL Server is a proprietary relational database management system developed by Microsoft. It is a full-featured database management system that can be used for a variety of purposes, including data warehousing, online transaction processing, and business intelligence.
Microsoft Access is a relational database management system developed by Microsoft. It is a desktop database management system that is designed for small to medium-sized businesses. Access is easy to use and has a user-friendly interface. However, it is not as powerful as other database management systems, such as MySQL or SQL Server.
Oracle is a proprietary relational database management system developed by Oracle Corporation. It is a full-featured database management system that can be used for a variety of purposes, including data warehousing, online transaction processing, and business intelligence. Oracle is a powerful database management system, but it is also expensive.
In conclusion, the most open source DBMS is MySQL. It is free to use, easy to use, and has a large community of users and developers.