Question

Sound waves have relatively long wavelengths. We can hear people around the corner before we can see them. Which wave behavior does this illustrate?

Answer 1

I'd say diffraction since sound waves can bend around objects like corners. Let's say you're in the hallway and you can hear sound coming from a door. The sound waves diffract around the door and spread out into the hallway, making it possible for you to hear.  
Also, you can hear it before you see it because light waves are shorter than sound waves and hardly diffract around doors.