Which of the following is considered open source software?

PostgreSQL is considered open source software because it is freely available to the public for use, modification, and distribution. This means that anyone can use PostgreSQL without paying for a license, and developers can access its source code to modify it according to their needs. The open source model encourages community collaboration and contributions, resulting in a robust and continually evolving database management system.

In contrast, options like Oracle Database, Microsoft Windows, and Adobe Photoshop are proprietary software products. These applications require a paid license for use, and their source code is not available for public modification or distribution. This distinction is crucial in understanding the differences between open source and proprietary software in terms of accessibility, flexibility, and community support.