Answer: Doppler effect is greater when the source approaches the observer.
Explanation:
Doppler effect always deal with observer relation with frequency of wave. Â Successive wave crest emitted from closer position towards observer than previous wave crest, since source of wave approaching towards observer. Since time between remittance of successive crest towards observer reduced, this leads to increase in frequency. So Doppler effect will be higher when source approaches towards observer and its reduced when source moves away from observer.