Final answer:
The correct option for the important part of a play's setting is D. lighting, as it influences mood, suggests locations, and enhances storytelling within the production.
Step-by-step explanation:
An important part of a play's setting is the incorporation of lighting. The use of lighting in a play is a powerful tool that not only enhances visibility for the audience but also sets the mood, establishes time of day, and even suggests locations without the use of physical sets. For instance, a scene might call for a character to be in a jail cell, and the lighting designer can create the essence of that setting with a tightly focused rectangle of light with shadows to represent the bars.
This demonstrates how lighting goes beyond mere illumination; it becomes an integral part of the storytelling and overall production design. In contrast, while characters are fundamental to a play, they are more aligned with the narrative than the setting. The audience also plays a role in the dynamic of a play but does not constitute as part of the setting. Therefore, the correct option in this context, focusing on the elements that directly influence the establishment of a play's setting, would be D. lighting.