The figure below shows how two computers communicate with each other. The interface between the two computers represents which of the following types of communication systems? Select all that apply. 

A.  Full duplex
B.  Half duplex
C.  Simplex
D.  Parallel
E.  Serial

To start, let's first understand the following:

1. The difference between simplex, half duplex, and full duplex
   • Simplex is a communication channel that sends information in only one direction
   • Half duplex is where multiple parties can communicate with each other, although not simultaneously
   • Full duplex is where multiple parties can communicate with each other and it may occur simultaneously
2. The difference between parallel and serial communication
   • Parallel communication is where multiple data bits are transported over multiple channels simultaneously
   • Serial communication utilizes a single channel to send data bits on after the other

In our diagram, we have both a transmitter and receiver on the two computers. Each transmitter is connected to the other computer's receiver. Therefore, when information is being sent from one computer to the other, it only takes place in a single TX to RX wire. Therefore, this is serial communication.

Since there are a set of two serial wires to communicate back and forth between the two computers, data transmission can thus occur simultaneously. Therefore, this system is also represents a full duplex communication system.

Therefore, our final answer is A and E, full duplex and serial.