Which is NOT a peripheral device?

Concept

A peripheral device is any auxiliary device that the CPU works with for input, output, storage, or communication — anything in the system that sits outside the core processing unit and its main memory. The CPU and the main memory (RAM) form that core, so they are never classed as peripherals; everything else attached to the system is.

Applying it here

Sort each item by whether it belongs to the CPU/main-memory core or is an auxiliary device the CPU works with:

• Main memory chip — this is RAM, the computer's primary working storage that the CPU reads from and writes to directly. It is part of the core, NOT a peripheral.

• Visual display unit — the monitor, an output device the CPU sends results to, so it is a peripheral.

• Modem — a communication device the CPU uses to exchange data with a network; whether built in or attached, it is auxiliary to the core, so it is a peripheral.

• Pen drive — a removable secondary-storage device, auxiliary to the core, so it is a peripheral.

Cross-check

Three of the four items (display, modem, pen drive) are auxiliary devices the CPU communicates with, and the computer can still boot and run without any one of them. Only the main memory chip is part of the processing core and is essential to running any program — confirming it is the one that is NOT a peripheral.