The process of modulation involves varying the properties of a wave in order to transmit information.
So in this case we're transferring the sound coming through the microphone on the back of a radio wave.
Into the question now:
at P, all that we have is the signal from the microphone. It's a sound wave which has been converted into an electrical signal, and the other waves which we're required to label are radio waves. In general, the frequencies of light, even in the form of radio waves, will be much higher than the frequencies of sound. So if we're trying to pick a waveform which will match the sound, we'll probably pick B, as its frequencies seems the lowest.
At Q, all the we're receiving is the 'carrier wave', the wave that will be altered to carry the sound wave. In this case it is a radio wave. The wave hasn't yet reached the modulator, so what we're looking for is a flat, uniform wave, which in this case will be A.
At R, the wave has been altered to carry the signal. The answer will be C, as you can see that the waveform has been altered to 'carry' the original sound sample on top of the radio waves being emitted. You can imagine that if you multiplied the waves in A and B together, you might get something that looks like C.
At S, the wave has been demodulated to form the original electrical signal from the microphone, so we're back to B again.