What type of device is usually considered an Audio Input Device?

An audio input device is specifically designed to capture sound and convert it into a format that a computer can process. A microphone serves this function by picking up audio waves generated by sounds like voice or musical instruments and converting them into electrical signals. This allows the computer to receive and interpret sound for various applications, from voice recognition to audio recording.

In contrast, the other options do not fulfill the role of capturing sound. A monitor displays visual output, a printer produces physical copies of documents, and a scanner converts physical documents into digital images. None of these devices are designed to input audio signals into a computer, which firmly establishes the microphone as the appropriate answer in this context.