What type of transmission device allows two-way communication but only one direction at a time?

The correct choice is half-duplex. This type of transmission device is designed to support two-way communication, meaning that both parties can send and receive information. However, it allows only one party to transmit data at a time. This capability is commonly found in devices like walkie-talkies, where one user speaks while the other listens, and the roles can be reversed, but not concurrently.

In contrast, full-duplex systems facilitate simultaneous two-way communication where both parties can send and receive messages at the same time, such as in typical telephone conversations. Simplex refers to a one-way communication channel, where data can only flow in one direction, like in a broadcast scenario. Multipoint implies a connection involving multiple endpoints, typically emphasizing the role of connecting several devices rather than the directionality of communication.