Julian downloaded some of the music he created to play for his friend. Julian's friend wore headphones to listen to the music. A set of headphones is:

an input device.

an output device.

a data storage device.

a type of storage.

A set of headphones is an output device

Explanation:

an output device is electronic hardware device that is used to showcase/produce the input been made into an electronic system usually a computer system. examples of such devices are speakers, monitors printers and scanners.

A set of headphones is an output device because it reproduces the music been downloaded (input) into the electronic/computer system been used for the downloads. while an input device is the means through which the music was downloaded into the computer system that the headphone will be connected to reproduce the music.

Answer: An output device

Explanation:

Headphones are the hardware output device and it use in the purpose of listening audio and music and watching movies without any interruption and distributing others.

It can used in the computer system by plugging it in or connecting it with the speakers.

We can connect headphones to the computer system which basically result in output as sound in the headphones and then it is not needed to use USB headphones.