Which of the following is not a technique for FM demodulation?

Communication Engineering, aka TeleCommunication Engineering covers topics such as basic circuit design to strategic mass developments. A telecommunication engineer is responsible for designing and overseeing the installation of telecommunications equipment and facilities, such as complex electronic switching systems, and other plain old telephone service facilities, optical fiber cabling, IP networks, and microwave transmission systems. Telecommunications engineering also overlaps with broadcast engineering. (Source: Wikipedia)