Which of the following is an example of a Client-Server setup?

The choice indicating a computer accessing a database on another server exemplifies the Client-Server setup effectively. In this configuration, the computer acts as the "client," which makes requests for data or services, and the "server," which hosts the database, responds to these requests. This model underlines a clear separation of roles; the client is the interface that users interact with, while the server manages resources and provides the necessary data or services to the clients. This interaction is fundamental in many networking and payment processing environments, improving efficiency and scalability.

In contrast, the other scenarios present different setups. For instance, two software applications running on the same machine indicate a local setup without distinct clients and servers. A standalone payment processing machine operates independently and does not involve communication with another server for processing. Similarly, a web application running on a local network might suggest some level of client-server interaction but falls short of defining roles clearly, as it could be designed to work in a more peer-to-peer manner or without the classical client-server dichotomy. Therefore, the first option captures the essence of the Client-Server model most accurately.