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 in distortion.
• B
  $m > 1$ will result in overlapping of both sidebands,resulting in the 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 the carrier signal is greater than the amplitude of the message signal,resulting in distortion.