The correct answer is: inversely proportional to the square of its distance from the source.
Intensity is a measure of the power per unit area of a sound wave. The power of a sound wave is proportional to the square of its amplitude, which is the maximum displacement of the air molecules from their equilibrium position. The distance from the source of a sound wave is inversely proportional to the area of a sphere with that distance as its radius. Therefore, the intensity of a sound wave is inversely proportional to the square of its distance from the source.
In other words, if you double the distance from the source, the intensity of the sound will be one-fourth as much. If you triple the distance, the intensity will be one-ninth as much. And so on.
This is why it is often difficult to hear someone who is speaking to you from a distance. The sound waves from their voice have to travel through a lot of air to reach you, and by the time they do, they have lost a lot of their intensity.
The inverse square law is a general principle that applies to many different types of waves, including sound waves, light waves, and electromagnetic waves. It is a consequence of the fact that waves spread out in all directions from their source. The further you are from the source, the more spread out the waves are, and the less energy they have per unit area.