Introduction:
A pre-emphasis circuit is used in audio communications to improve the signal-to-noise ratio (SNR) and enhance the overall quality of the audio signal. It is commonly employed in broadcasting and telecommunications systems to compensate for the frequency response characteristics of the communication channel.

Explanation:
The correct answer is option 'C', which states that a pre-emphasis circuit amplifies higher audio frequencies. Let's understand why this is the correct answer and how it provides extra noise immunity in audio communications.

1. Frequency Response of Communication Channels:
Communication channels, such as radio or telephone lines, often have frequency response characteristics that attenuate higher frequencies more than lower frequencies. This can result in a decreased SNR and reduced intelligibility of the transmitted audio signal.

2. Pre-Emphasis Circuit:
A pre-emphasis circuit is designed to compensate for the frequency response characteristics of the communication channel. It introduces a deliberate boost in the higher audio frequencies before transmission.

3. Amplifying Higher Audio Frequencies:
By amplifying higher audio frequencies, the pre-emphasis circuit increases their relative power compared to the lower frequencies. This boost helps to overcome the attenuation experienced by these frequencies during transmission.

4. Signal-to-Noise Ratio Improvement:
The amplification of higher audio frequencies by the pre-emphasis circuit helps to improve the SNR. When the audio signal is received at the other end, a corresponding de-emphasis circuit is employed to reverse the frequency response changes introduced by the pre-emphasis circuit.

5. Noise Immunity:
The pre-emphasis circuit provides extra noise immunity by effectively increasing the power of higher audio frequencies, which are more susceptible to noise. Since the higher frequencies carry more information and are crucial for intelligibility, boosting them helps to overcome the noise floor and improve the overall quality of the received audio signal.

Conclusion:
In audio communications, a pre-emphasis circuit amplifies higher audio frequencies to compensate for the frequency response characteristics of the communication channel. By doing so, it improves the SNR and provides extra noise immunity, resulting in enhanced audio quality and intelligibility.