The correct answer is D.

Concept:

The modulation index in AM is always kept less than 1 to avoid distortion or clipping of the message signal. When m > 1, the carrier undergoes a phase reversal of 180° and the envelope of the modulated signal no longer contains the actual message signal, resulting in a distorted output from the envelope detector.

Explanation:

• When m > 1, the amplitude of the message signal exceeds the amplitude of the carrier signal.
• This causes over-modulation, leading to envelope distortion where the envelope detector cannot faithfully recover the original message.
• Additionally, m > 1 results in overlapping of both sidebands, causing loss of information.
• Therefore, option D is the correct answer.