Which of the following is a common example of a secondary storage device used to store data long-term?

Introduction / Context:
Computer systems use several types of storage. Primary storage (main memory or RAM) holds data that the CPU is currently using, while secondary storage keeps data long term even when the power is off. Understanding which devices serve as secondary storage is important for system design and everyday usage. This question asks you to identify which option is a typical secondary storage device.

Given Data / Assumptions:

• Secondary storage devices are non-volatile and store data persistently.
• The options include an ALU, a disc, and a projector.
• We consider discs to include hard disks, SSDs (solid-state drives), and optical discs like DVDs.
• We assume standard textbook definitions of primary and secondary storage.

Concept / Approach:
Secondary storage devices include hard disk drives, solid state drives, CDs, DVDs, Blu-ray discs, and other media that hold data long term. Discs are classic examples, used to install operating systems, store documents, and archive backups. The Arithmetic Logic Unit (ALU) is a CPU component that performs calculations and does not store data long term. A projector is an output device that displays images and presentations and is not used for data storage. Therefore, the only correct example of secondary storage among the options is the disc.

Step-by-Step Solution:
Step 1: Define secondary storage as non-volatile, long-term data storage. Step 2: Recognise that discs such as hard disks and optical discs are designed to store large amounts of data persistently. Step 3: Identify the ALU as a processing component, not a storage device. Step 4: Identify a projector as an output device that shows images, not a device that stores data. Step 5: Note that All of the above cannot be correct because ALU and projector are not storage devices. Step 6: Select disc as the correct example of a secondary storage device.

Verification / Alternative check:
Basic computer fundamentals books describe the memory hierarchy: registers and cache at the top, main memory (RAM), and then secondary storage such as hard drives and optical discs. ALU and control units appear in separate sections on CPU components, and projectors appear in sections on output devices. Real world usage also confirms that users save files to disks and discs, not to ALUs or projectors. This evidence supports disc as the correct answer.

Why Other Options Are Wrong:
Arithmetic Logic Unit (ALU): Performs arithmetic and logical operations; it processes data but does not store it long term. Projector for displaying images: Outputs visual content but has no role in persistent data storage. All of the above devices: Incorrect because only the disc is a secondary storage device; ALU and projector are not.

Common Pitfalls:
Learners sometimes confuse storage with any hardware inside or attached to the computer. To avoid this, focus on the device's main purpose: does it store data when the power is off? If yes, it is likely secondary storage. If it only processes, displays, or transmits data, it belongs to another category such as processing, output, or communication devices.

Final Answer:
A common example of a secondary storage device is a Disc (such as hard disk or optical disc).