Final answer:
The term for software that can be freely used, adapted, and contributed to by anyone is 'Open source'. It promotes collaboration and improvement of the software by its users under an open copyright license.
Step-by-step explanation:
The term defined as software that allows users to use and adapt it for any purpose, with the possibility for the public to participate in further development, is c) Open source. Similar to open textbooks, open-source software is typically made available under an open copyright license and can be freely modified and distributed.
These projects are often collaborative endeavors that benefit greatly from community contributions. For example, a program under a Creative Commons Attribution license would allow use, modification, and distribution as long as the original creator is credited. This principle supports innovation and continuous improvement by a diverse user base.