For AM, value of modulation index should be always \((\mathrm{A})<1\) (B) \(=1\) (C) \(>1\) (D) 0

Amplitude Modulation (AM) is a method of encoding information in a carrier wave by varying its amplitude according to the modulation signal (message signal). In this process, the frequency and phase of the carrier wave remain constant, and only the amplitude is modified according to the modulation signal. Step 2: Define Modulation Index

In amplitude modulation, the modulation index (m) is a dimensionless parameter that indicates the extent of amplitude variation of the carrier wave caused by the modulating signal. It is given by the ratio of the amplitude of the modulating signal (A_m) to the amplitude of the carrier wave (A_c): \[ m = \frac{A_m}{A_c} \] Step 3: Analyze Modulation index limits

For a proper amplitude modulation, the modulation index (m) should have a value between 0 and 1 (0 ≤ m ≤ 1). If the modulation index is less than one (m < 1), it implies that the modulation is called undermodulation or low modulation which means the amplitudes of sidebands will be small, meaning the message signal will not be transmitted efficiently. When the modulation index is equal to 1 (m = 1), it is called full or 100% modulation. In this case, the maximum power efficiency is achieved without distortion in the transmitted signal. If the modulation index is greater than 1 (m > 1), it is called overmodulation. In this case, the sidebands will overlap, which causes distortion, and it is not suitable for practical communication purposes. Step 4: Conclusion

Based on our understanding of amplitude modulation and the value of modulation index for proper transmission, we can conclude that the value of modulation index should always be: (A) < 1 Which means the modulation index should always be less than 1 to have a proper amplitude modulation transmission without distortion.