Light from an object moving away is shifted toward the red end of the visible spectrum.
This phenomenon is called redshift. As a light source recedes, the wavelengths reaching an observer are stretched, so spectral features move toward longer wavelengths. In the visible band, longer wavelengths lie toward red; an approaching source produces the opposite effect, blueshift, toward shorter wavelengths and the violet-blue end.
Astronomers measure redshift by comparing familiar spectral lines from stars, galaxies, or gas with their laboratory wavelengths. The amount of shift can reveal an object’s motion along the line of sight. Redshift is one reason spectroscopy is such a powerful tool for studying astronomical objects.
Not every redshift is caused by ordinary motion. Doppler redshift comes from relative motion, while gravitational redshift results from light escaping a gravitational field and cosmological redshift is associated with the expansion of space. The quiz’s moving-object wording specifically points to the Doppler interpretation.