Final answer:
The device that transmits signals from a computer onto a network is the Network Interface Card (NIC).
Step-by-step explanation:
The device that sends signals from a computer onto a network is the Network Interface Card (NIC).
A router is used to connect multiple networks and route traffic between them. A modem modulates and demodulates signals for communication over telephone lines. A switch connects devices within a single network and manages data flow between them. However, the NIC is a hardware component that allows a computer to connect to a network and communicates with other devices on that network.