Concept:The main internal storage device where the computer keeps its data and files.Explanation:The system unit is the main case of the computer that holds its internal components.The Hard Disk Drive (HDD) is typically installed inside the system unit and serves as the primary storage medium for the operating system, software, and user files.Options A, B, and D are removable or external storage media, not the main storage located within the system unit.Answer:C. HDD