What component converts data to a format suitable for transmission over phone lines?

The correct choice is the modem, which is essential for converting digital data from a computer into an analog signal suitable for transmission over phone lines. This process involves modulation, where the modem modulates the digital signal into a format that can travel over the analog infrastructure of the phone system. When the signal reaches its destination, another modem demodulates it back into digital data for the receiving computer. This conversion is crucial for ensuring that data can effectively travel over telephone networks, which are inherently designed for analog signals.

The router, on the other hand, is responsible for directing data packets between different networks and does not perform signal conversion for transmission over phone lines. Similarly, a switch operates within a single network to connect devices and manage data traffic but does not convert data formats for transmission. A firewall serves a completely different purpose by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules, rather than interacting directly with data transmission formats.

Thus, among the available options, the modem specifically fulfills the role of converting data to a suitable format for phone line transmission, making it the correct answer to the question.