What type of duplex allows for data transmission in only one direction at a time?

Half-duplex is the correct answer because it defines a communication method where data transmission can occur in only one direction at any given time. In half-duplex mode, a device can either send or receive data, but not both simultaneously. This is akin to a walkie-talkie system where one party must wait for the other to finish speaking before they can respond.

In contrast, full-duplex allows for simultaneous two-way communication, where both devices can send and receive data at the same time, much like a telephone conversation. Simplex refers to a one-way communication channel where signals can travel only in one direction, like a keyboard sending input to a computer, but it’s not suitable in the context of devices that can switch roles between sending and receiving. Duplex, by itself, is a general term and does not specify the directionality of data transmission, making it an insufficient choice when defining transmission modes.