The correct option is B Modulation
The method of super imposing message signal on carrier wave is Modulation. Many a times when audio signals are transmitted over large distances, the frequencies that lie within the range of 15 Hertz to 20 KiloHertz has very small signal power and cannot be transmitted by antenna. High frequency signals on the other hand can be sent over large distances with small dissipation in power. Thus the message signals are super imposed with high frequency wave- called carrier wave which are generally undamped in nature. When the message signal is superimposed on a carrier wave, the
resulting wave gets all the characteristics of the message signal and thus becomes useful for communication.
Option D is correct.