Which of the following is an example of a version control system?

A version control system is a tool that helps developers track changes to files over time, enabling them to collaborate more effectively and manage different versions of their projects. SVN, or Subversion, is a widely recognized example of such a system. It allows multiple users to work on the same project while maintaining a history of changes, making it easy to revert to previous versions if needed.

In contrast, the other options are technologies used for different purposes: PHP is a server-side scripting language used for web development, HTML is the standard markup language for creating web pages, and CSS is a style sheet language used for describing the presentation of a document written in HTML or XML. While these technologies are essential for programming and web design, they do not provide the functionalities of version control that SVN offers, focusing instead on the creation and styling of content.