Which of the following is an example of a relational database?

MySQL is a widely recognized example of a relational database management system (RDBMS). It organizes data into structured tables with predefined relationships among them, adhering to the principles of the relational model. This structure allows users to easily create, read, update, and delete data through the use of Structured Query Language (SQL).

In a relational database like MySQL, data integrity and accuracy are maintained through the use of constraints like primary keys, foreign keys, and indexes. These elements ensure that the relationships between different data sets are logically consistent and enforce rules that govern how the data is stored and accessed.

The other options represent different types of database technologies. MongoDB and Cassandra are examples of NoSQL databases, which are designed to handle unstructured or semi-structured data and provide scalability and flexibility. Redis, on the other hand, is an in-memory data structure store that's often used as a cache or message broker, again falling outside the relational database model.