Which of the following is used for modulation and demodulation ?

Modem is used for both modulation and demodulation.
Modulation is the process of changing the characteristics of a signal (referred to as the carrier wave) to transmit information across a medium (such as cables, air, or fiber optic cables). The purpose of modulation is to encode the data onto the carrier wave so that it can be transmitted efficiently and accurately.
On the other hand, demodulation is the process of extracting the original data from the modulated carrier wave at the receiving end. Demodulation reverses the modulation process and allows the receiver to retrieve the original information sent by the transmitter.
A modem (short for modulator-demodulator) is a hardware device that performs both modulation and demodulation. It acts as a bridge between the digital signals from a computer or network and the analog signals used for transmission through a telephone line, cable, or wireless medium. The modem modulates the digital signals from the computer into analog signals that can be transmitted over the network or phone line. At the receiving end, it demodulates the analog signals back into digital signals that can be understood by the computer or network.
In summary, a modem is used for modulation and demodulation, enabling the transmission and reception of data over various communication channels.