Which of the following is considered an input hardware device?

The keyboard is classified as an input hardware device because its primary function is to allow users to input data and commands into a computer or other electronic system. It consists of keys that represent letters, numbers, and various functions, which when pressed, send data to the computer for processing.

In contrast, the other options serve different functions: a monitor displays output from the computer, a printer produces physical copies of digital documents, and speakers output sound. Therefore, the keyboard's role in entering data makes it the correct answer among the choices provided.