Frequency Modulation:
Frequency modulation (FM) is a modulation technique used in telecommunications to encode information on a carrier wave by varying its frequency. It is widely used in applications such as radio broadcasting, two-way radio communication, and radar systems.

Explanation:
In frequency modulation, the amplitude of the carrier wave remains constant. This means that option 'B' is the correct answer.

Key Points:
To understand why the amplitude of the carrier wave remains constant in frequency modulation, let's explore the key points of FM:

1. Carrier Wave: The carrier wave is a high-frequency sinusoidal waveform that carries no information. It serves as the medium through which the modulating signal is transmitted.

2. Modulating Signal: The modulating signal is the input signal that contains the information to be transmitted. It can be an audio signal, video signal, or any other form of data.

3. Frequency Deviation: In frequency modulation, the frequency of the carrier wave is varied according to the amplitude of the modulating signal. This variation is known as frequency deviation.

4. Frequency Modulation Index: The frequency modulation index (also known as the modulation index or modulation depth) determines the extent of frequency deviation. It is the ratio of the peak frequency deviation to the frequency of the modulating signal.

5. Phase Deviation: In FM, the phase of the carrier wave is also affected by the modulating signal. However, the phase deviation is relatively small compared to the frequency deviation.

Advantages of Constant Amplitude:
Maintaining a constant amplitude in FM offers several advantages:

1. Improved Signal Quality: By keeping the amplitude constant, FM can effectively combat amplitude variations caused by noise or interference, resulting in a higher signal quality.

2. Increased Signal-to-Noise Ratio: The constant amplitude of the carrier wave allows for better detection and extraction of the modulating signal, leading to an improved signal-to-noise ratio.

3. Reduced Interference: Constant amplitude simplifies the demodulation process, making it less susceptible to interference from other signals or distortions.

4. Compatibility: Maintaining a constant amplitude makes it easier to transmit and receive FM signals using various receiver configurations, ensuring compatibility across different systems.

Conclusion:
In frequency modulation, the amplitude of the carrier wave remains constant while the frequency is varied according to the modulating signal. This constant amplitude characteristic allows FM to provide improved signal quality, increased signal-to-noise ratio, reduced interference, and better compatibility.