What type of hardware is primarily used to play audio output?

The primary hardware used to play audio output is headphones. They are specifically designed to convert electrical signals into sound, allowing users to listen to audio content privately. Headphones come in various styles, including over-ear, in-ear, and on-ear types, and they can connect to devices through wired or wireless methods. Their main purpose is to deliver high-quality sound directly to the user's ears, making them the ideal choice for personal audio playback.

In contrast, a monitor primarily displays visual output rather than audio, while a scanner is used to digitize physical documents or images. A keyboard serves as an input device for data entry but has no functionality related to audio output. Thus, headphones stand out as the correct answer due to their dedicated role in audio playback, emphasizing their importance in the realm of audio devices.