The amount of lighting could set a brighter or darker tone, as could the colors used to portray the mood. Less lighting could mean the play has a more serious topic, where natural lighting could lend a happier theme.
D is the correct answer.
In the badly drawn diagram attached, you will find the <em>yellow</em> represents wavelength, which is the distance between identical points on consecutive waves.
The <em>green</em> represents the amplitude, which is the distance between the origin (middle) and crest/trough (top/bottom)