The correct answer is D. Windows 2000.
Windows 2000 is a network-ready operating system that was designed with client-server networks in mind. It offers a number of features that make it well-suited for this type of network, including support for multiple protocols, security features, and the ability to manage multiple users and resources.
MS DOS is a single-user, single-tasking operating system that is not well-suited for client-server networks. It does not support multiple protocols, does not have strong security features, and cannot manage multiple users and resources.
Windows 95 and Windows 98 are also single-user, single-tasking operating systems. They offer some improvements over MS DOS, such as support for multiple protocols and better security features. However, they still do not have the ability to manage multiple users and resources, which makes them unsuitable for client-server networks.
In conclusion, Windows 2000 is the best operating system for implementing a client-server network. It offers a number of features that make it well-suited for this type of network, including support for multiple protocols, security features, and the ability to manage multiple users and resources.