Final answer:
A de-emphasis network is used in FM receivers to restore lower audio frequencies that were proportionally attenuated during the transmission process, balancing the audio frequency response.
Step-by-step explanation:
The audio shaping network added at an FM receiver to restore proportionally attenuated lower audio frequencies is known as a de-emphasis network. In FM transmission, higher audio frequencies are pre-emphasized (boosted) at the transmitter to improve the signal-to-noise ratio for those frequencies. When the signal is received, the de-emphasis network attenuates the high frequencies to their correct level relative to the lower frequencies, creating an even response across the audio spectrum and restoring the original sound quality.