Explanation:

A peripheral device is an auxiliary device that connects to and works with a computer. These devices extend the computer's capabilities and can be either internal (e.g., a graphics card in a PCIe slot) or external (e.g., a USB mouse). All these devices ultimately connect to the motherboard, which is the main circuit board of the computer. Option (c) 'called a peripheral device' is the most accurate and encompassing term. Option (a) 'called an external device' is too narrow, as many devices connected to the motherboard are internal (e.g., RAM, CPU, internal hard drives). Option (b) 'called an adjunct device' is not a standard computing term. Option (d) 'must connect using ribbon cable' is incorrect because while some older internal devices (like IDE hard drives) used ribbon cables, many modern devices use different connectors like SATA, PCIe, or USB.