Answer:
D. The pitch does not change.
Step-by-step explanation:
When the source of sound moves away, the pitch drops. and when the source of sound approaches the pitch rises. This is called'Doppler effect'.
Here the person a and the sound horn are both in the same vehicle. Which means their relative velocity is zero. So the horn is neither approaching nor receding the person A
If the source is approaching

If the source is receding

Hence the right answer is option D. The pitch does not change