Which of the following defines free software?

Free software is primarily defined by the freedom it provides users to run, modify, and share the software. This means that users can access the source code, alter it to suit their needs, and even redistribute their modifications to others. These freedoms are central to the concept of free software and support a collaborative environment where improvements and customizations can be made freely.

While there are other aspects related to copyright and distribution, the ability for users to modify the software is the core distinguishing feature. Although software can also be free in the sense that it costs nothing, the key aspect related to free software is the user's freedom to modify and enhance it.

The other options do not capture the essence of free software. Software that must be purchased does not align with the free philosophy, as it implies restrictions and a barrier to access. Software that is always online does not inherently relate to the concept of free software, as there are many types that can be installed and run offline. Lastly, while software with no copyright restrictions might suggest a level of freedom, the definition of free software more specifically stresses the freedoms related to use and modification rather than the absence of copyright altogether.