In amplitude modulation, the modulation index m, is kept less than or equal to 1 because
(a) m > 1, will result in interference between carrier frequency and message frequency, resulting into distortion.
(b) m > 1 will result in overlapping of both side bands resulting into loss of information.
(c) m > 1 will result in change in phase between carrier signal and message signal.
(d) m > 1 indicates amplitude of message signal greater than amplitude of carrier signal resulting into distortion.