The method of communication in which transmission takes place in both directions, but only one direction at a time is called

A. Simplex:
Simplex communication is a method of communication where transmission occurs in only one direction. It does not support two-way communication.

B. Four wire circuit:
A four-wire circuit refers to a communication channel that uses separate wires for transmission and reception, allowing for simultaneous two-way communication. However, this option does not describe the scenario where only one direction is allowed at a time.

C. Full Duplex:
Full duplex communication is a method where transmission can occur simultaneously in both directions. It allows for two-way communication simultaneously, which is not the case in the given scenario.

D. Half Duplex:
Half duplex communication is the correct answer. In this method, transmission can take place in both directions, but only one direction can occur at a time. The communication switches between the two directions, allowing communication in both directions but not simultaneously.

Therefore, the correct answer is:
D. Half Duplex