Which device is necessary for converting the signal received on coaxial cable to data usable by a computer?

The correct choice is a modem because it serves the critical function of converting signals from coaxial cable, which typically carries data in the form of analog signals, into digital data that can be used by a computer. This process is essential for establishing internet connectivity since most internet service providers (ISPs) deliver data through coaxial cables. The modem translates the incoming analog signals into a digital format, which can then be processed by computers and other digital devices.

In contrast, a router is primarily used to direct data packets between different networks and devices, managing the traffic within a network rather than converting signals. A switch connects multiple devices within a local area network (LAN) and facilitates communication between them but does not convert signal types. A hub is an older networking device that connects multiple Ethernet devices, making it operate at a simpler level by broadcasting data to all connected devices without any data manipulation or conversion. Each of these devices plays a role in networking but does not fulfill the specific function of signal conversion that a modem does.