Final answer:
The unique element of tone in a stage production is sound, as it encompasses vocal delivery, background noises, sound effects, and music.
Step-by-step explanation:
The element of tone that is unique to a stage production is sound. While all elements like word choice, plot, and conflict contribute to the tone, sound is uniquely influential in a stage setting because it includes the vocal delivery, background noises, sound effects, and music that can only be fully experienced in a live performance. These auditory elements, combined with visuals and performance, define the overall tone in a way that cannot be replicated through reading the script alone.