The correct answer is D. Windows 2000.
Windows 2000 is a network-ready operating system that can be used to implement a client-server network. It offers a number of features that make it well-suited for this purpose, including support for multiple protocols, security features, and the ability to manage multiple users and resources.
MS DOS is a single-user operating system that is not well-suited for use in a client-server network. Windows is a multi-user operating system that can be used in a client-server network, but it does not offer the same level of support for multiple protocols and security features as Windows 2000. Windows 98 is a consumer-oriented operating system that is not well-suited for use in a client-server network.
In conclusion, Windows 2000 is the best operating system to choose for implementing a client-server network. It offers a number of features that make it well-suited for this purpose, including support for multiple protocols, security features, and the ability to manage multiple users and resources.