In which one of the following the early FM push to talk telephone system was used ........

Explanation:

Answer: a
 The early FM push to talk telephone systems were used in the Half Duplex. The Half Duplex is a type of communication in which the information or data can back and forth between two devices. In this type of communication, each device can send and receive the data, but only one device can transmit the data at a time. So the correct answer will be the B.