Answer: The correct answer is "the Doppler effect".
Explanation:
Doppler effect is the phenomenon in which there is a relative motion between the observer and the source. It may result to apparent change in the frequency.
Scientists use the Doppler effect to understand the universe by determining the motion of the object.
Red shift: It is a phenomenon which results from the Doppler effect. Light changes as the object in space (such as star or galaxies) moves towards from us then the light is shifted to blue end. In this case, the light is shifted to shorter wavelength.
Therefore, the correct option is (B).
