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

Options

A. poor selection of modulation index (selected `0ltmlt1`)
B. poor bandwidth selection of amplifiers.
C. poor selection of carrier frequency
D. loss of energy in transmission

Explanation

To solve the problem of why a male voice sounds like a female voice after modulation and transmission, we can analyze the options provided and determine the correct reason step by step.

### Step-by-Step Solution:

1. **Understanding Modulation**:
- Modulation is the process of varying a carrier signal in order to transmit information. In this case, the information is the male voice.

2. **Identifying Voice Frequencies**:
- Male voices typically have fundamental frequencies ranging from about 85 Hz to 180 Hz.
- Female voices usually range from about 165 Hz to 255 Hz.

3. **Analyzing the Problem**:
- The problem states that a male voice is received as a female voice. This implies that the frequency characteristics of the voice have changed during transmission.

4. **Evaluating the Options**:
- **Option A: Poor selection of modulation index**:
- The modulation index should be between 0 and 1 for effective modulation. If it is within this range, it does not cause a change in voice characteristics. Thus, this option is incorrect.

- **Option B: Poor bandwidth selection of amplifier**:
- If the bandwidth of the amplifier is not properly selected, it may allow frequencies that are higher than the original male voice frequencies to pass through. This can result in the male voice being altered to sound like a female voice. This option is plausible.

- **Option C: Poor selection of carrier frequency**:
- The carrier frequency should be selected based on the modulating signal. A poor selection of carrier frequency would not directly cause a change in the voice characteristics. Thus, this option is incorrect.

- **Option D: Loss of energy of transmission**:
- Loss of energy can occur during transmission, but it does not specifically cause a change in the perceived pitch or gender of the voice. Thus, this option is incorrect.

5. **Conclusion**:
- The most likely reason for the male voice sounding like a female voice after modulation and transmission is due to **poor bandwidth selection of the amplifier** (Option B). This can cause the frequencies of the male voice to be altered, leading to the perception of a female voice.

### Final Answer:
The problem is due to **poor bandwidth selection of the amplifier**.