Answer:
Option a: One program calls another to request a task of some kind.
Step-by-step explanation:
A client/server application is a two-tier architecture where a computer machine act as a client and another as a server. It always involves the activity where a client machine (e.g. user computer) make request to a remote server for information.
One example would be our web browser which send http request to a remote web server and the server will response with the requested info that will be assembled and displayed in the web browser of the client computer.