Which of the following is used for modulation and demodulation ?

Modulation is the process of converting digital data from a computer into an analog signal suitable for transmission over a communication line, such as a telephone line. Demodulation is the reverse process, converting a received analog signal back into digital data that a computer can understand.

A device called a modem, whose name is actually a shortened combination of "modulator" and "demodulator," performs exactly these two functions. When sending data, the modem modulates the computer's digital signal into an analog form; when receiving data, it demodulates the incoming analog signal back into digital form.

A gateway is a device or software system that connects two different networks and translates between their protocols, but it does not perform modulation or demodulation of signals. A protocol is simply a set of agreed rules governing how data is formatted and transmitted, not a physical device. Multiplexing refers to combining multiple signals so they can share a single transmission channel, which is a different function from converting between digital and analog forms.

Exam tip: remember that the very name "modem" is built from "modulator-demodulator," which directly tells you its function whenever a question asks about converting between digital and analog signals.