The correct answer is "the letter a. speed."
The Doppler Effect is used in Police Car Radars in determining and measuring speed. It refers to an increase/decrease in the frequency of a sound, or a light, or other known waves as the origin and as the spectator move towards/away from one another. This effect creates drastic changes in the pitch, which is highly noticeable in a passing siren, and is also seen by the astronomers in the redshift.