Final answer:
For Roosevelt's listeners on the radio, gesture would have been the least important public speaking element.
Step-by-step explanation:
When Franklin D. Roosevelt delivered his fireside chats on the radio, the least important public speaking element for his listeners would have been gesture. Since the audience could not see him, his physical movements and gestures would not be visible. Instead, Roosevelt relied heavily on his tone, inflection, and enunciation to convey his message and connect with the listeners.