Which input device is primarily used to transfer audio to a computer as digital files?

The microphone is the correct choice because its primary function is to capture sound waves and convert them into electrical signals. These signals can then be processed by a computer and converted into digital audio files. This process is essential for tasks such as voice recording, audio input for video conferencing, and other applications that require the capture of sound.

The other devices serve different purposes. A speaker is used for outputting audio, not inputting it. A scanner is designed for digitizing physical documents and images, converting them into digital formats like PDFs or images. A webcam captures video and may include audio capabilities, but it is primarily focused on video input rather than audio. Therefore, the microphone stands out as the dedicated input device for transferring audio to a computer.