Which of the following devices is considered a peripheral?

A peripheral device is typically defined as any external device that connects to and works with a computer to either input data or output data. In this context, the mouse serves as an input peripheral device, allowing users to interact with the computer interface through pointing and clicking actions.

The mouse is specifically designed to enhance user interaction with the computer, making it a quintessential example of a peripheral device that connects to the CPU but is not a part of its core functionalities.

On the other hand, the CPU, hard drive, and motherboard are essential components of the computer's architecture. They are critical for processing and storage tasks and are considered internal components rather than peripherals. The CPU, or central processing unit, performs calculations and processing tasks; the hard drive stores data; and the motherboard serves as the main circuit board that connects all components. Hence, they do not fit the definition of peripheral devices.