What type of communication channel allows signals to travel simultaneously in both directions?

The correct choice is full-duplex, which refers to a communication channel that allows signals to travel simultaneously in both directions. This means that both parties in a communication process can send and receive data at the same time. For example, a telephone conversation is a common example of full-duplex communication, where both people can talk and listen simultaneously without waiting for the other to finish speaking.

In contrast, half-duplex allows for data transmission in both directions, but not at the same time. This means that while one party is sending a message, the other must wait until the transmission is complete before it can respond. Simplex communication only allows data to flow in one direction, with no possibility of a return signal from the receiving end. While duplex is a broader term that can refer to systems capable of carrying signals in two directions, it does not specify that both directions can occur at the same time, which is a defining feature of full-duplex communication.