Step 1: Recall the definition of an output device.
An output device is a hardware component that receives processed data from the computer and presents it to the user in a readable or usable form.
Examples include:
\[
\boxed{\text{Monitor, Printer, Speaker, Plotter, Projector}}
\]
Step 2: Analyse the options.
• (A) Keyboard -- Incorrect. It is an input device used to enter text and commands.
• (B) Scanner -- Incorrect. It is an input device that converts printed documents into digital form.
• (C) Monitor -- Correct. It displays processed information from the computer and is therefore an output device.
• (D) Microphone -- Incorrect. It is an input device used to record sound.
Step 3: Final conclusion.
Since a monitor displays the output generated by the computer,
\[
\boxed{\text{Monitor is an output device.}}
\]
Hence, the correct option is \(\boxed{(C)}\).