You're right, the correct answer is diffraction.
When a sound wave bends around an obstacle and remains audible even though the source is obscured, it has diffracted around the barrier.
The other options do not describe this phenomenon:
- Compression implies reducing the wavelength of the wave, which does not happen here.
- Reflection means the wave bounces off the obstacle, but in this case the wave bends around it.
- Refraction means the wave changes direction when passing from one medium to another, but there is no medium change described here.
So diffraction is the appropriate term to describe a sound wave bending around an obstruction while remaining audible.
Well done! I'm glad I could help confirm the right answer.