Which of the following is considered a common input device?

A scanner is considered a common input device because it takes physical documents or images and converts them into a digital format that computers can process. This functionality is essential for transferring data from the real world into a computer system for further use, such as editing, storing, or sharing.

In contrast, while the other options are important components of computer systems, they all serve roles that are not classified as input. A monitor displays output from a computer, allowing users to visualize data and information. A printer produces physical copies of digital documents, making it an output device as well. Speakers convert digital audio signals into sound, serving another form of output. Understanding the role of input and output devices is essential when studying computer hardware and how it interacts with user input and output functions.