In which scenario does redshift primarily occur?

Redshift primarily occurs when an object is moving away from the observer. This phenomenon is a consequence of the Doppler effect, which describes how the frequency of waves changes in relation to an observer who is moving relative to the source of the waves. As an object moves away, the wavelengths of light emitted by that object are stretched, resulting in a shift towards the red end of the electromagnetic spectrum.

This effect is particularly evident in astronomy, where light from distant galaxies is often redshifted, indicating they are receding from us. This observation supports the expansion of the universe, as most galaxies appear to be moving away from our point of view. Thus, the correct answer highlights the relationship between the motion of distant objects and the observed shift in their light spectrum.