Which of the following best describes a modem's function?

A modem's primary function is to allow internet access by modulating and demodulating signals for communication over phone lines, cable systems, or satellites. When a modem connects to an internet service provider (ISP), it converts digital data from a computer into a format suitable for transmission over these lines, and vice versa, enabling data exchange between the internet and your device. This vital role is essential for accessing online resources, browsing the web, and communicating through various internet services.

The other options describe functionalities unrelated to a modem's purpose. Typing documents pertains to word processing software and user interaction rather than any hardware function of a modem. A modem is not a physical part of a printer, as printers serve different functions related to outputting documents on paper. Lastly, monitoring document formats is primarily the role of software applications that handle and display document data, not the function of a modem.