Answer:
A) Propagation of pressure fluctuations in a medium
B) air is the medium in which the wave is transported, 
Step-by-step explanation:
Part A. 
A sound wave is a longitudinal oscillation of the molecules that forms in a material medium, they can be solid, liquid or gases, therefore the wave propagates in the same direction as the oscillation of the particles.
 
The most correct answer is: 
* Propagation of pressure fluctuations in a medium 
 
Part b 
air is the medium in which the wave is transported, otherwise it cannot propagate