The correct answer is C. Zero crossing detection.
A phase locked discriminator, slope detection, and product detector are all techniques for FM demodulation.
Zero crossing detection is not a technique for FM demodulation. It is a technique for AM demodulation.
In zero crossing detection, the signal is sampled at the zero crossings of the carrier wave. The difference between the samples is then used to demodulate the signal.
This technique is not suitable for FM demodulation because the frequency of the carrier wave is much higher than the frequency of the modulating signal. This means that the zero crossings of the carrier wave are very close together, and it is difficult to accurately measure the difference between them.
As a result, zero crossing detection is not a reliable technique for FM demodulation.