What did Richard Stallman develop?

Richard Stallman developed a free Unix-like operating system known as GNU (GNU's Not Unix). The GNU Project, initiated by Stallman in 1983, aimed to create a completely free software operating system to counteract the growing trend of proprietary software. This project emphasized the importance of software freedom, allowing users to run, modify, and distribute software as they wish.

GNU was significant in the history of software development because it laid the groundwork for subsequent operating systems and strongly influenced the open-source movement. It incorporated a variety of tools, libraries, and components that allowed for the creation of a complete operating system environment. Eventually, when the Linux kernel was developed, it was combined with GNU components, leading to what is commonly referred to as "GNU/Linux." This combination fully realized Stallman's vision for an operating system that prioritized user freedom.

The other options do not accurately reflect Stallman's contributions or initiatives. The focus on free software and open-source principles is a defining aspect of the GNU Project, setting it apart from commercial or proprietary software models.