Question

What are the external parts of a computer system and which are output devices and which are input devices

Answer 1

I'd question the wording, However:

Monitor, Output device.
Keyboard, Input device.
Mouse, Input device.
Speakers Output Device.
Microphone, Input device.
Printer, Output device.
Scanner, input device.

Devices do not have to explicitly be one or the other e.g. a printer with a scanner, or headphones with a microphone.