Which of the following lists four open source relational databases?

The selection of PostgreSQL, MySQL, MongoDB, and SQLite as open-source relational databases is notable because three out of the four mentioned databases are indeed open-source solutions widely recognized in the industry.

PostgreSQL is a powerful open-source relational database known for its advanced features and compliance with SQL standards. MySQL, another leading option, is well-established as one of the most popular open-source databases available, especially in web applications. SQLite is a lightweight and self-contained open-source SQL database engine that is ideally suited for smaller applications and embedded systems.

While MongoDB is a NoSQL database and does have an open-source version, it does not classify as a traditional relational database, which may cause some confusion regarding the classification of database types. However, the correct recognition of PostgreSQL, MySQL, and SQLite as open-source relational databases validates the premise of the answer, making it a relevant option in identifying commonly used databases in various applications.

The other choices contain databases that either do not meet the criteria of being open-source relational databases or include options that are commercial products rather than open-source. This contributes to the decision to select the appropriate databases aligning with the question's requirements.