A. Absorption
-It's the process in which any objects can be absorbed, thus in this case, sound waves.
I'm not completely sure if this is correct, but the other three choices don't make sense too much, then again this is on the subject of biology.
-Diffraction involves light and distance, not sound
-Reflection has to do with the many subjects which reflect, this can't relate to sound
-and transmission involves the transmitting or movement of a certain object to another, in biology's case, cells or pathogens.
That's why I believe it's A.