Among the following options, which device is not considered an external storage device for a computer system?

Introduction / Context:
Computer systems use many different types of storage devices. Some are external storage devices that can be removed and transported easily, while others are internal components that provide working memory for the system. This question asks you to identify which device among the options is not classified as an external storage device, which is important for understanding basic hardware categories.

Given Data / Assumptions:

• The list includes CD-ROM, DVD-ROM, pen drive, and RAM.
• External storage typically refers to removable media or devices used for backup and data transfer.
• We assume a standard personal computer environment.

Concept / Approach:
CD-ROM and DVD-ROM are optical discs that can be inserted into and removed from drives, and pen drives are USB based flash storage devices. All three are commonly used for storing and transferring data between systems and are considered external storage media. Random Access Memory RAM, however, is an internal component mounted on the motherboard and is used as main memory for currently running programs. It is volatile and not used as external storage.

Step-by-Step Solution:
Step 1: Classify CD-ROM as a removable optical disc used for storing software, music, and data. It is clearly an external medium. Step 2: Classify DVD-ROM similarly as a removable optical disc with higher capacity than a CD, again an external storage medium. Step 3: Identify a pen drive as a USB flash drive, which is a small, portable external storage device used to move files between computers. Step 4: Recognise that RAM modules are plugged into slots on the motherboard inside the system unit and function as primary memory, not as external or portable storage. Step 5: Conclude that RAM is the only option that does not belong to the category of external storage devices.

Verification / Alternative check:
Ask yourself which of these devices you could safely remove while the computer is turned off and carry to another computer to read data. You could remove a CD-ROM, DVD-ROM, or pen drive and use them elsewhere. RAM, on the other hand, is internal and not designed for frequent removal for data transfer. It does not retain data when power is off and is not used as portable storage, which confirms that it is not an external storage device.

Why Other Options Are Wrong:
CD-ROM: This is wrong as the non external device because CD-ROM discs are removable optical media, a classic example of external storage.
DVD-ROM: This is wrong because DVD-ROM discs, like CDs, are designed to be inserted and ejected and are used for external storage and distribution of data.
Pen drive: This is wrong because pen drives are specifically designed as portable external storage devices that plug into USB ports.

Common Pitfalls:
Some learners confuse internal versus external with primary versus secondary storage. RAM is primary internal memory, while CD, DVD, and pen drive are secondary external storage media. Another pitfall is assuming that anything which stores data must be considered external, but the word external here refers to physically removable media rather than any memory device. Paying attention to context avoids this confusion.

Final Answer:
The device that is not an external storage device is Random Access Memory RAM.