Which of the following is an input device?

The keyboard is indeed an input device because it is specifically designed to allow users to input data and commands into a computer. When a user types on a keyboard, it sends signals to the computer, translating the keystrokes into digital information that the computer can process. This interaction is fundamental to user-computer communication, enabling the entry of text, commands, and instructions.

In contrast, the other options listed are all output devices. A monitor displays visual output from the computer, showing information and graphics generated by the system. A printer produces a physical copy of digital documents and images, enabling users to have tangible output. Lastly, a speaker outputs audio signals generated by the computer, providing sound for media and notifications. Thus, the keyboard stands out distinctly as the correct answer in the context of identifying input devices.