If you relate the doppler effect on sound to the color of light waves, a police car would be the color (a. yellow b. red c. green d. blue) as it approaches and the color (a. red b. violet c. blue d. green) as it recedes away from the observer.

The Doppler effect says that the observed wavelength is shortened

when the source is approaching the observer, and lengthened when

the source is receding from the observer.

All we can say about the police car and its siren is that the sound/color

has a shorter wavelength as it approaches us, and a longer wavelength

as it recedes.

Any of these pairs of answers would say that:

  A -- A

  C -- A

  D -- A

or

  D -- D

For each of these pairs, the second wavelength is longer than the first one.

(We don't know which pair is the actual one, because we don't know the actual color of the police car.)