What type of device allows users to input data into a computer system?

The correct choice, a mouse, is a device that directly allows users to input data into a computer system. It serves as a pointing device, enabling users to interact with the computer's graphical user interface (GUI) by moving the cursor, clicking on icons, and selecting options. The mouse translates hand movements into signals that the computer can interpret, ultimately allowing for precise navigation and command execution.

Other options, while related to computer systems, do not serve the primary function of inputting data as a mouse does. A scanner, for example, converts physical documents and images into digital format but is not used for interactively inputting commands or data in real time. A router is responsible for directing data traffic between networks and does not input data from users. A hard drive is a storage device that holds data rather than inputting it. Thus, the mouse is uniquely positioned as the device that actively allows user input in the context of typical computer use.