What best characterizes a simplex radio system?

A simplex radio system is defined by its ability to communicate on a single frequency in one direction at a time. This means that one party can transmit while the other listens, but they cannot send and receive messages simultaneously on the same channel. This characteristic makes simplex systems straightforward and easy to manage, as they don't require complex frequency switching or additional equipment to handle bidirectional communication. In practical terms, this setup is often used in scenarios where simple, single-channel communication is sufficient, such as in certain types of walkie-talkies or base-to-mobile communications.

The other options describe features or systems that are either more complex or designed for different communication protocols. For instance, systems that can receive multiple transmissions simultaneously relate to multiplexing, whereas using different frequencies for sending and receiving pertains to duplex systems, which allow for two-way communication on separate channels. Finally, the mention of requiring additional hardware would imply a system that is not inherently simple, which directly contradicts the essence of a simplex radio system.