Which of the following devices is used specifically to convert signals for telephone line transmission?

A modem is a device specifically designed to convert digital signals from a computer into analog signals suitable for transmission over telephone lines, and vice versa. This process is known as modulation and demodulation, hence the name "modem" (modulator-demodulator). When a computer sends data to be transmitted, the modem modulates the digital signal into an analog format that can travel over the traditional phone line. On the receiving end, it then demodulates the incoming analog signal back into a digital format that the computer can understand.

In the context of telecommunications, the modem plays a crucial role in enabling internet connections and facilitating communication through analog infrastructure, such as telephone lines. Other devices like routers, switches, and wireless access points (WAPs) serve different purposes, such as directing data traffic within networks or providing wireless connectivity but do not specifically handle the conversion of signals for phone line transmission.