Answer - C. Demodulation
The diode detector detects the AM signal envelope, which is the most basic type of amplitude modulation AM signal detector.
Although its ease of use has been the primary factor in its broad adoption, its performance is inferior to that of other types of AM detectors and demodulators, especially in distortion levels.
The fundamental AM diode signal detector is commonly utilized in RF circuits for signal level detection and AM envelope detection.
The signal's envelope is output by the AM diode detector, which is an envelope detector. As a result, the output of the diode detector or demodulator can be proportional to the size of the envelope of an amplitude-modulated signal.
Hence, Option (C) is the correct answer.