In amplitude modulation, the modulation index m is kept less than or equal to 1 because
(a) m > 1, will result in interference between the carrier frequency and message frequency, resulting in distortion
(b) m > 1 will result in overlapping of both sidebands resulting in loss of information
(c) m > 1 will result in a change in phase between the carrier signal and the message signal
(d) m > 1 indicates the amplitude of message signal greater than the amplitude of the carrier signal resulting in distortion
Due to the occurrence of overmodulation , this leads to higher chances of sideband overlapping and fading (loss of information). If the value of u is approximately equal to 1, it results in a distortion f carrier wave.
Thus option (b) and (d) are the right answers.