Which type of DBMS is primarily stored in main memory?

The Main Memory Database Management System (MMDBMS) is specifically designed to store data primarily in a system's main memory, rather than on traditional storage devices like hard disks. This allows for significantly faster data access and processing speeds, as accessing data in memory is much quicker than retrieving it from disk storage.

This type of database is particularly useful in scenarios where high-speed data processing is crucial, such as in real-time data analytics, complex transaction processing, or applications requiring immediate responsiveness.

In contrast to MMDBMS, relational, object-oriented, and distributed DBMSs may use disk-based storage and often involve more complex mechanisms for data retrieval and manipulation that can introduce latency. Consequently, MMDBMS stands out in environments where performance is paramount, making it the correct answer in this context.