D

Step 1: Understand the concept of Doppler effect.
The Doppler effect refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.

Step 2: Identify the key variables affecting the Doppler shift:
- The frequency of the wave produced by the source (Option A) definitely affects the observed frequency.
- The velocity of the source (Option B) impacts how quickly the wavefronts are generated, thus affecting the observed frequency.
- The velocity of the observer (Option C) alters how quickly the observer encounters the wavefronts, thereby influencing the observed frequency.

Step 3: Analyze the dependency on distance (Option D).
The Doppler shift does not depend on the distance from the source to the listener. While the intensity of the sound may decrease with distance, the observed frequency due to the Doppler effect solely depends on the relative velocity between the source and the observer, not their distance apart.

Conclusion: Therefore, the correct answer is D.