ANS:A - PSK

Coherent detection requires the phase information of the carrier phase. This can be achieved either by sending a carrier from a transmitter or by using a local oscillator at the receiver that is phase locked with the transmitter. Thus there exists a phase difference between the two which has to be estimated and corrected for detection. In non-coherent detection, no phase information is required. Since PM is required to continually change the phase of the carrier, therefore, non-coherent detection which does not require phase information can't be used to detect it. To detect PM one must use phase information as the message is present in phase information.