Final answer:
In a computer network, the node that requests and uses resources from other nodes is known as a client. It operates within the client-server model, contrasting with the server that provides the resources.
Step-by-step explanation:
In the context of a computer network, the node that requests and uses resources from other nodes is known as a client. This is in contrast to a server, which is a node that provides resources to other nodes. The client-server model is a fundamental concept in network architectures where the client makes a service request from the server, which then fulfills the request. For instance, when you use a web browser (the client) to access a website, it requests the website's data from a web server.
To clarify the options given: a server (b) is the providing node, a directory server (c) is a specialized server that manages resources such as names or addresses within a network, and a host (d) can be either a server or a client in network terminology, referring broadly to a networked computer.