Answer: Option C
Explanation: The intensity of the sound wave is directly related to the "volume" of the sound.
A) The density of the medium: As the density increases, the volume of the sound decreases at a higher speed, this is why if the source of sound is in other room the volume decreases, this is because the walls are "absorbing" the intensity.
B) The amplitude of the wave is directly related to the intensity, as bigger is the amplitude, bigger is the intensity.
D) The sound decreases with the distance squared, this means that two different persons that are at differnt distances of a speaker, will experience a diferent volume.
C) The source is the thing that is creating the sound, the "actual" source does not have an impact in the volume, two different speakers can produce equivalent sound waves with equivalent volume/intensity.