A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to

A female voice has higher frequency than a male voice. Hence, the frequency of the modulated signal is more than the frequency of the original signal. This can be due to poor bandwidth selection of amplifiers. Note that twice the frequency of the modulated signal is equal to the bandwidth. Hence, (b) is correct.

To change the modulation index, we need to change the frequency of the carrier wave. We can still retrieve the frequency of the message signal upon demodulation. Hence, option (a) and (c) are incorrect.

Loss in energy isn’t due to change is frequency but because that energy is spread. Only, a bit of the wave strikes the receiving end. Hence, (d) is incorrect.