Final answer:
The term used to describe an OS designed mainly for sharing network resources is Network Operating System (NOS), which is part of the evolution from proprietary systems to open-source ones like Linux.
Step-by-step explanation:
The term used to describe an operating system designed mainly to share network resources is a) Network Operating System (NOS). These systems are critical for managing network resources efficiently and are often found at the core of network operations and management. The history of operating systems includes the evolution from tightly controlled proprietary systems, such as those by IBM and Microsoft, to more open systems like UNIX.
Over time, the openness of UNIX paved the way for Linux, an open-source operating system that became popular for its configurability and cost-effectiveness, often being used in servers and internet routers. Today, Linux dominates the operating system landscape for many networked systems, including the world's top supercomputers. This shift has had a significant impact on computing, with Linux replacing expensive proprietary systems and ending the monopoly of companies like Microsoft on the OS market.