Which device serves as an interface between a user and the computer?

The correct answer is the input device, as it directly serves as the primary means by which a user interacts with a computer. Input devices are designed to take user input and convert it into a format that the computer can understand. Common examples include keyboards, mice, touchscreens, and microphones. These devices facilitate a two-way communication channel, allowing users to enter data and commands to the system, which is essential for effectively operating and utilizing a computer.

In contrast, a router primarily functions to direct data traffic between networks and facilitates communication over the internet rather than serving as a direct user interface. The operating system, while crucial for managing hardware and software resources and providing an environment for user interaction, is not itself a device but rather the software that runs on the computer. The power supply is a hardware component that provides electrical power to the computer and does not play a role in user interactions. Thus, the input device stands out as the key interface that enables user engagement with the computer system.