Doppler Effect Steps:

• Source (ambulance siren) emits constant frequency; observer stationary.
• As ambulance approaches at constant speed, relative velocity reduces wavelength, increasing heard frequency.
• After passing, ambulance moves away, increasing wavelength, decreasing heard frequency.
• Post-pass acceleration increases receding speed, causing heard frequency to decrease further over time.

Why C is correct:

• Doppler formula: approaching, $f' = f \frac{v}{v - v_s} > f$ (higher frequency); receding, (lower frequency), where is sound speed, source speed.

Why the others are wrong:

• A: Reverses both shifts; approaching increases frequency, receding decreases it.
• B: Approaching increases frequency, does not decrease.
• D: Receding decreases frequency, does not increase.

Final answer: C