Answer: a)ASK
Step-by-step explanation:Amplitude shift keying (ASK) is the the modulation of the signal on the basis of amplitude. The variation that occur in the amplitude of the carrier signal makes it most prone towards the noise and change the amplitude.
Other options are incorrect because FSK(frequency shift keying) and PSK(phase shift keying) are not susceptible as frequency and phase do not get change due to noise impact respectively .QAM(quadrature amplitude modulation) will also get affected due to noise but not to the extent of ASK.Thus, the most affected signal will be of option(a).