Which of the following describes simplex radio systems?

In a simplex radio system, communication is one-way at any given time, meaning only one user can transmit while the other must listen. This is characteristic of simplex systems, where the channel is dedicated to either transmission or reception, but not both simultaneously. This structure makes it efficient for certain types of communications, much like a walkie-talkie system where one person speaks while the other listens, and they must take turns to communicate.

Alternatives offered, such as simultaneous transmission by both users or the need for multiple frequencies or channels, do not accurately reflect the functionality of simplex systems. Simplex is distinctly defined by its single-channel, one-directional communication capability.