which communication channel allows the sending of information in one direction only?

Communication channels are classified by the direction in which data can travel between two devices.

Simplex mode allows data to travel in only one direction at all times; one device can only transmit and the other can only receive, with no possibility of the roles reversing. A common example is a radio broadcast, where the signal flows from the transmitter to the receiver and never the other way.

The other modes all allow communication in both directions, differing only in timing. Half duplex mode allows data to travel in both directions, but only one direction at a time, similar to a walkie-talkie where users take turns speaking. Duplex and full duplex mode (these refer to the same concept) allow data to travel in both directions simultaneously, as happens in an ordinary telephone conversation.

When a question asks for a channel that sends data in one direction only, with no reverse flow at any time, that description matches simplex mode specifically, since every other mode listed permits two-way communication.