Which of the following is a secondary storage device?

Concept: A computer's storage hierarchy is organized by proximity, speed, capacity, and persistence. Primary or internal memory supports immediate CPU work, whereas secondary storage retains larger amounts of data for long-term use even when power is removed.

Application: A hard disk is a non-volatile mass-storage device: it records files and programs on magnetic platters and preserves them across power cycles.

Contrast:

• Cache is fast CPU-adjacent memory for recently used data; it is part of the working memory hierarchy, not a long-term mass-storage device.

• Register is storage inside the CPU for operands, addresses, and control state needed immediately.

• ROM is non-volatile semiconductor memory used mainly for firmware or fixed startup instructions; in the standard memory hierarchy used by this question, it is internal memory rather than a secondary storage device.

Cross-check: IBM's storage hierarchy lists CPU registers and caches above RAM and disk, while its secondary-storage overview includes hard disk drives. This places a hard disk on the persistent, high-capacity side of the hierarchy.

Result: Therefore, the secondary storage device among the offered values is Hard Disk.