The correct answer is the last option (D).
Set design is an element of tone directly related to a stage production. By means of the set design, we help the audience situate on the right setting (place and time) for the development of the story.
For example, if we performed Shakespeare's Romeo and Juliet in a supermarket and the characters were wearing everyday clothes, probably the audience would get confused. We need to put the characters in context and that is the function of the set design.