Final answer:
Mac OS X is based on NextStep, which uses a UNIX kernel. UNIX's commercialization led to various proprietary versions, while the development of Linux provided a free and flexible alternative that has become widely adopted in many areas of computing. 2) UNIX is correct option.
Step-by-step explanation:
Mac OS X is built on NextStep, which is an operating system based on the UNIX kernel. The history of UNIX begins with its initial development by researchers at AT&T Bell Labs and the University of California, Berkeley.
Over time, however, UNIX became commercialized, leading to the creation of different proprietary versions by various companies.
In contrast, Linus Torvalds, dissatisfied with the cost of UNIX, developed the Linux kernel, which was free and could be easily configured by a programmer, leading to widespread use in various applications, from PC operating systems to file servers and internet routers.
Additionally, because of its open-source nature, Linux has been adopted by organizations like NASA for high-performance computing, essentially revolutionizing the market and even outperforming proprietary supercomputers.