Which of the following describes a modem?

A modem is a critical piece of hardware used to facilitate data transmission between different networks, specifically connecting a local network to the internet. Modems convert digital signals from a computer or network into analog signals that can be transmitted over telephone lines or cable systems, and vice versa. This functionality enables communication over various types of infrastructure, allowing computers and devices to access internet services.

In this context, understanding the role of a modem is essential, as it serves as a bridge between the local area network (LAN) and the wide area network (WAN) that is the internet. The other options do not accurately describe a modem's purpose: connecting devices in a local area network refers more to networking equipment like switches or routers, graphic design software is an entirely different category of tools used for creating visual content, and protocols for internet communication define rules and standards for data exchange but do not describe hardware.