Which radio transmission technique subdivides the allocated frequency range into smaller units called channels?

The correct answer is Frequency Division Multiplexing (FDM). This technique involves dividing an available frequency spectrum into multiple smaller frequency bands or channels, each used for a different signal or data stream. By allowing multiple transmission signals to coexist simultaneously without interference, FDM efficiently utilizes the frequency spectrum.

FDM is commonly applied in various communication systems, including radio broadcasting, where each station transmits its signals on a designated frequency channel, preventing overlaps and enabling simultaneous transmissions. Each channel occupies a specific frequency range within the larger spectrum, allowing for multiple communications to occur without contention for the same frequency.

The other techniques listed serve different purposes. Time-division multiplexing allocates different time slots for signals to use the same frequency, while spread spectrum techniques spread the signal across a wide bandwidth to minimize interference. Code Division Multiple Access assigns unique codes to different users sharing the same frequency band, allowing their signals to coexist. Each of these methods has its unique applications and benefits in the realm of communications.