Correct Option: C

When the carrier to noise ratio is slightly less than unity, the frequency of spike generation is small, and each spike produces individual clicking sound in the receiver. But, when the carrier to noise ratio is further decreased so that the ratio is moderately less than unity, the spikes are generated rapidly and the clicks merge into sputtering sound. This phenomenon is known as threshold effect in FM. and the level of threshold depends upon the frequency deviation and power level of frequencies radiated at the receiver.