Final answer:
OPTION D.
The Doppler effect leads to a higher pitched siren sound as an emergency vehicle approaches due to the compression of sound waves, resulting in shorter wavelengths and higher frequencies.
Step-by-step explanation:
The Doppler effect causes a siren's pitch to increase as it approaches you because the sound waves are compressed, or pushed closer together, as the source moves towards you. This compression leads to a decrease in wavelength and an increase in frequency, which our ears interpret as a higher pitch. So, the correct answer to your question is D. Sound waves are pushed closer together, decreasing wavelength and increasing frequency.