Microsoft DOS is not a network operating system; it is a single-user operating system.

Introduction:
This question belongs to the SSC CGL category and it asks to identify the network operating system that is not popular among the given options. The correct answer is option 'D', which is Microsoft DOS.

Explanation:
Network Operating System:
A network operating system (NOS) is a software platform that allows multiple computers to communicate, share resources, and run applications over a network. It provides various services such as file sharing, printer sharing, security, and network management.

Popular Network Operating Systems:
1. NetWare: NetWare is a network operating system developed by Novell. It was one of the most popular network operating systems in the 1990s. NetWare provided advanced features like file and print services, directory services, and security. However, its popularity declined with the emergence of other network operating systems.

2. Windows NT Server: Windows NT Server is a network operating system developed by Microsoft. It was one of the early versions of the Windows Server operating system. Windows NT Server provided features like file and print services, user and group management, and security. It was widely used in businesses and organizations.

3. Windows XP: Windows XP is a client operating system developed by Microsoft. While it is not a dedicated network operating system like NetWare or Windows NT Server, it can be used in a network environment. Windows XP provided basic networking capabilities like file and printer sharing, but it was primarily designed for individual users and not as a dedicated network operating system.

Microsoft DOS:
Microsoft DOS (Disk Operating System) is an operating system developed by Microsoft. It was one of the early operating systems for personal computers. However, it is not a network operating system. MS-DOS was primarily used as a single-user operating system and did not have built-in networking capabilities. It was later replaced by Windows operating systems, which provided networking features.

Conclusion:
In conclusion, the network operating systems mentioned in the options are NetWare, Windows NT Server, and Windows XP. The operating system that is not a popular network operating system is Microsoft DOS. MS-DOS was primarily used as a single-user operating system and did not have built-in networking capabilities.