A natural setting..  is the answer
        
                    
             
        
        
        
Answer: C. Audience
Explanation:
        
             
        
        
        
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)