The correct answer is: The frequency of the sound the person observes is higher than it was, but the wavelength does not change.
As the police car moves towards the person, the observer experiences the Doppler effect. The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. When the source of the sound (police car's siren) moves towards the observer, the observed frequency increases, resulting in a higher-pitched sound. However, the wavelength of the sound remains the same since it's determined by the source. This effect is commonly heard in real-life situations, such as the change in pitch of a passing vehicle's horn as it approaches and then moves away from an observer.