Which of the following is an output device ?

An input device sends data or instructions from the user into a computer, while an output device sends processed information out of a computer to the user in a visible, printed, or projected form.

Checking each option against this rule:

• Projector: enlarges and casts the computer's video output onto a screen, delivering it as visible output.

• Keyboard: sends key-press signals from the user into the computer for processing.

• Mouse: sends pointer-movement and click signals from the user into the computer for processing.

• Scanner: sends a captured image of a document from the user into the computer for processing.

Cross-check: only the projector carries data out of the computer for the user to view; the keyboard, mouse, and scanner all carry data into the computer, confirming they are input devices.

Hence, the projector is the output device among the given options.